SlimmJimm

The separate Venus will have more ports, but you should be able to do everything with the builtin GX that you need.
 
The Multi GX also have 2 Ve.Bus ports not show in this picture. So the big difference I see are the 2 extra relays (the multi has one) and 1x Ve.Direct vs 2x in the Venus.

Oh that's a bit more difficult to answer. The Cerbo has a lot of really nice ports for doing all sort of things, but unless you have a boat or water tanks or things you want/need to measure, the GX model inverter with the built-in board is going to suffice. And it is still the fastest (it is faster than the Venus-GX, which is based on a Beaglebone black, and it is even a tad faster than the Cerbo, although the Cerbo is actually plenty fast). You can make this decision on cost: If an external GX works out cheaper, by all means go for that.
All the GX models have two... but it's really only two physical plugs. It's basically RS485, so it's just one bus. The two plugs are to make it easy to daisy-chain in larger setups. On a single-phase single-inverter install, you'll only use one anyway. It is useful to have that second one around, cause you can plug the mk3-usb in there... (just not for firmware updates, for firmware updates you must disconnect the GX).

No. The GX models have the control board built in, and a neat little LCD panel on the front that displays some basic info.
The EasySolar-II GX has both the GX control-board and an MPPT built in. The Multiplus-II GX has only the control board, and you'd need an external MPPT (but you can plug it into the board inside the inverter, there are two VE.Direct ports on it that you can use).
The MK3-USB is only really required for firmware updates. Configuration can be done via any GX device (CCGX, Venus-GX, Cerbo-GX, the built-in board, all of them) by using VRM two-way communication and the remote ve.configure feature on the VRM website.
You can do firmware updates directly from the GX device BUT it requires some skills. If you know your way around a Linux/Unix system, you should be okay. Otherwise, either ask someone to do it for you, or buy/borrow the mk3.
For normal day-to-day operation you don't need the mk3.
I really have a bit of an issue with suppliers that still ship the CCGX, even when the customer clearly doesn't need it AND asks for Wi-Fi. It's the oldest and the most expensive of the control units. If you don't need the LCD screen, then buy a cheaper GX unit. The Venus-GX and the Cerbo-GX are both cheaper, and both of them have built-in Wi-Fi.
I'd go with the GX model. It has much more CPU power on its control board than the much older CCGX (quad core ARM A7 at 1.2Ghz vs a much older single-core ARM A8 at 600Mhz).