JustAsking

I would suggest watching more of Will Prowse's beginner videos

No need to replace a pure sine with any of the cheaper modified sine inverters. As was suggested get some panels and a controller to keep batteries charged during day time. Charging from grid is perhaps the cheaper option specifically if you have a stable grid without long outages.

Forgot to mention that your Panel must produce at least 29V (Normal operating conditions) for the PWM to do its job. 
Solar panel connects to PWM 24V Charger input... The battery output goes to the battery.
Setup the bulk absorption to 28.8V and float to 27.4V  (double check those on the battery specsheet)
The inverter does not care as long as there is voltage, the PWM charger does not care as long as there is a battery and sun. 
If power falls away then the battery will last a lot longer as the sun will keep it charged.
 
 

Miskien te streng, jammer. 😶

At first I thought to suggest as penance that you have to read a Jodi Picoult novel before you are allowed to read more firmware (😉) - then I noticed that I actually referred to a 3000kVA fictional inverter to power a couple of residential CCTV cameras... so it is more appropriate that I stare blankly at some firmware for a while and then also read a Jodi Picoult novel for all the bad thoughts while staring at said firmware.
You sir are correct. Units matter. The end. 🙂

Know what you mean. Gets even more fun when having multilingual keyboard options and forgetting to change the language before starting your message..
but trying to set an example for political leaders the world over, I can unfortunately not blame anyone else or even my phone - mistake was all mine 😳
have a heart, we are still adjusting to the shock of having electricity for two days in a row and now you want us to use correct units and technical lingo. Je bent een strenge leraar (if I recall correctly from a previous post you have some non-Aussie lineage) 😉. 
... I actually just realised considering current world events (movements?) that "Units matter" can actually go in a whole different direction as a conversation and even if tongue-in-cheeck will likely get me banned, so back on topic and to upset the more technically inclined: @JustAsking 3 kVA is 3000VA...which is sort of like 3000 Watt (or 3kW) - it gives you an indication what the maximum load is that the inverter can power (you size an inverter for the peak load that you expect - not the average load).

I'm always amazed by the predictive text on 'smart' phones. You go back and read your message which isn't what you intended..(but hopefully the recipient is smart enough to reverse the predictive text algorithm and get the message!)

Welcome Mr M H. (you must be a very patient person with telesales people if posting your phone number in a public forum 😲 😉).
Any case, adding to what my learned forum colleague posted - for your described need (just the goodies in the garage) battery voltage is probably fairly irrelevant except possibly for the garage motor. Kudos on checking loads first - just keep in mind that the killa watt is unlikely to show you the inrush current when the motor starts up (that could easily go up to 7 x the motor run rating for a very short while which could trip up a smallish inverter). Have a look at this short thread for a bit more (24V 1.2kVA inverter struggled - BUT a number of issues could have affected the outcome... BTW it appears that the same poster also tried the 2.4kVA (?) inverter without luck).
Other than the DVR and possibly the garage motor I would expect now a days that most of the "appliances" you mention will have battery bacup already, so you could consider just upping the installed battery sizes (some systems will already be compatible with just replacing the 7Ah battery with 11Ah). You could also add a bigger dedicated (or two) power supply that has space for even bigger batteries (something like this). You win a bit by reducing the ineffiecient converting AC to DC (and total cost will be way below R35K). You will just need to check the different voltage requirements of the different systems ( @Richard Mackay is very pro DC, handy with a soldering iron and I think quite close to your neck of the woods so he could maybe give you some input on this 🙂). 
If this is not firmly included in your plans (i.e. at least having calculated number of panels etc. you will consider, or ideally already order with your system) I will not let included MPPT charge controller sway me in my purchasing decision (think of the number of semi-complete radio controlled airplanes, treadmills, golf clubs etc. that are gathering dust in the garage after "mmm that looks like a nice idea...one day I will get around to it.... now where is that advert for the classic land rover rebuild that just needs an engine and interior overall?"...). You should be able to fairly easily add solar later to roughly any system -  it is possibly just a bit more expensive (many variables though).
For this type of system wiring into the/a DB has two components - eskom electricity to the inverter and inverter produced electricity to what you want to power. One specific benefit of wiring into the DB is convenience (no extensions to trip over, use the usual light switches etc) at the cost of...well cost (sparkies are not cheap...for a reason). I have no idea what the CPS3000 install requirements are and I am not qualified to make specific recommendations so the more qualified will hopefully pipe up (?? @plonkster @Jaco de Jongh @Gerrie ..apologies to those not mentioned by name) but there will likely be a SANS (common sense?) requirement around limits of inverter to run of a "standard" plug. Most RSA plugs are 16A rated and probably run in a 20A protected circuit (MCB in DB). The CPS3000 is rated 3000VA with a 6000VA surge... using @Louisvdw's info you should see 3000VA/230A=13A... just makes it in for the plug rating... 6000VA/230V=26A... you may never approach these loads but the system can...so at the very least it is probably wiser for it's own incoming eskom power to have the unit on a dedicated circuit with correct wiring/circuit breakers etc. 
even DIY is not that difficult but give @Jaco de Jongh a shout (he  is likely running around more than usual at the moment but has built some UPS focussed systems). I am too lazy 😳 to hunt and crunch numbers but something like a 3000kVA multiplus ii and pylontech battery might put you in the ball park of the CPS price

Not sure what the 3Kv is?
But lets look at the other questions.
If your need is for a 300W usage then you need a 300W-500W inverter to cover some margin. There is nothing wrong with 24V or even 12V. If you go over 2000W then the amps start to get a bit high and you need very think cables, so then it is more economical to go for a 48V system. So for 500W I would say 12V or 24V will be perfect.
Here is how that works: P = V * I where P is the Power in Watts, V is Voltage and I is the current in Amps. So for 300W at 12V the current is 25A. That same 300W at 24V have a current of 12.5A  so you need half the cable size. (Cable size is rated to handle the current)
Lithium batteries are expensive but can handle more cycles (charge and discharge). If you are going to use it as energy storage that you use each day this is the way to go. 
For a UPS that just works when the power fails Gel Lead Acid will be fine and might save you some money. If you choose your system wisely you can swop to Lithium batteries later without having to change components. (You need something where you can change the charge voltages)
As to size you need to handle 300W x 8hours which is 2400Wh. If this is a 12V system then this is a 200Ah battery, or 100Ah battery for 24V. For Lead Acid (Gel) you should not use more than 50% so double that battery size (400Ah or 200Ah), while for Lithium you can use up to 80% (240Ah or 120Ah), but again add some margin for losses.
You might want to look at this tread. You might need a bit more battery power to get to 8 hours, but the rest will fit perfectly: