Something 100% original, matching nr's, fine shape, engine overhauled etc will cost quote a lot... so in view of that the price is quite decent, the twin head adds some big +++. Wanting something to show or something to ride ? Of course both, find your balance of satisfaction vs cash spent. Personally I am not hunting originality, just whatever I am happy loking at and riding.
As for repairs & parts, most maintenance parts don't cost that much individually ...but will have a big tendency to add up to quite a lot over time. Machining work, depends on the hour wages in your country. F.ex. start by asking a known good shop how much it costs to bore out cylinders and fit a timing side crank plain bearing. Cylinders, check that they are not bored out beyond max.
No documents, so never been registered legally in CZ? Assuming it's not a stolen bike or something, will there be problems (or impossible) getting CZ paperwork, quite an important 'detail', may depend of rules in your particular country. Can the seller be trusted if asked for details about the gearbox\engine internals, previous work done etc?
Do lots of research. A7A10's mechanical\engine etc everything can be fixed and parts available, good original tinware can be more difficult.
Good luck, tell us how it goes, welcome to ask detailed questions. And the english is very good.