Ursi

I've sent it via PM.

@Ursi About the fuse, you only really need one on the string. The reason is that the + and - wires of the string will always carry the same amount of current (Amps). Think of it as a closed loop that start of at the panels, goes through the combiner box to the inverter and then back to the combiner box, back into the panels. The current that originate in the panels always goes back into the panels. A fuse will blow (stop current flowing) if the current through it gets too high. When this happens the current will stop flowing on both the positive and negative wires. If you have two fuses, one on the positive and one on the negative wires, one of the two will blow first and stop the current from flowing through the other.
Regarding the SPD, It is a very good question. You had me thinking for a long time. I am no expert with this but here's what I think. (Someone please correct me if I am wrong)
A single pole SPD has two terminals. The one attaches directly to earth, the other attaches to a "live" wire that you want to protect from surges. If the voltage differential between the "live" terminal and earth reaches a certain value then it will "arch through" and allow the surge potential to dissipate to ground via the SPD (rather than your inverter or other expensive equipment).
If one of the wires (negative or positive) is grounded INSIDE the inverter then the voltage potential on that wire will always be the same as ground (0V). The voltage on the other wire will fluctuate. I suspect, that if this is the case then you would only require a single pole SPD. If on the other had you have a floating DC circuit were neither the positive or the negative wires are bonded to earth (inside the inverter/charge controller) then you would require a dual pole SPD.
NOTE: Don't ever ground your DC negative or positive wire unless the inverter/charge controller manufacturer explicitly state that you have to!

Hi Ursi,
I have a setup simular to yours. 13x 255W panels on the roof all in Series. Voc of the string at just below 500V. I also considered building my own  combiner box that include three things (all rated at over 500V).
1. DC Isolator (dual pole)
2. DC SPD
3. Fuse holder with fuse
The DC stuff rated at 500+ Volts are not that easy to source. You can't just walk into any shop and find it. There are places you can find them though. When you find them they are a LOT more expensive than the AC breaker etc. Since, I have mostly Microcare devices already I just closed my eyes and bought their combiner box that include all of the above. There are other solar shops that also supply combiner boxes that include all of the above.
From what I could gather (someone my correct me here) is that most combiner boxes only feature Type 2 SPD. The Type 1 is very expensive and mostly help with direct strikes (Is this correct?) Even so, if you have a direct strike you will most likely still loose a lot of equipment  (the type 1 SPD as well) not to mention roof tiles/bricks etc. That is why I have insurance.  You can protect your house against direct lightning strikes but you need to know exactly what your are doing and spend a lot of money in the process. Before I started the solar journey my house (as most in my area) did not have, and still do not have any type of direct lightning protection. I considered putting it in place but that would just inflate the cost of going solar. So in the end I just have a type 2 SPD on the DC side and a type 2 SPD on the AC side in the main DB. That should protect my investment against induced voltages of overhead lightning strikes or strikes that are close to my house.
I'm also interested to see what the other people on the forum has to say about it.
 

Hey Ursi, thanks for your question with the same inverter and 12x 260 panels I am also dealing with my shopping list. Best answer I can help you with at the moment: move to CPT much cheaper, I can speak from experience! I am not intending to use any surge protection. (we only get wind surges)but maybe I am wrong?
What does the rest of your shopping list look like and how many baths do you intend heating with your setup?