First of all, the topic´s name is not quite well chosen - english and direct question would have been better.
CuPlex-Question:
You only need the CuPlex Evo if you want 2-3 degrees better temps and the nice look. Otherwise the standard CuPlex is enough, I use it too and am very satisfied with its performance.
Also there are better modding possibilities... ;D
All the same you will need straight connections (= gerade Anschlüsse) in more than 99% of all cases.
I don´t know about the Dutch watercool "standards", but PnC connectors are very reliable and much easier to handle than any other connection. But if you like you can order 10mm connections instead.
There are only problems if you mix the types of hoses (= SChläuche) in your system, PnC Connectors mandatory use PUR hoses with 8/6mm diameter (outside/inside).
Twinplex GF4:
No, the connectors do not have to be the same than the ones for the CuPlex. I advise you to use the recommended AC-solution: TP Grafikkarte with connectors at the side. There is a special version, look for it in the shop.
TP Chipset:
Use straight connectors, like AC recommends.
Radiator:
Depends on how you want to install your system. It is mandatory that you install fans on the radiator if you install inside your case. In outside installation you can also use the radiator without extra fans, but you will not have good temps in your system then.
Eheim:
Use the 1046 like AC recommends, it works fine in all systems I know. Haven´t seen anything here which it couldn´t handle.
Accessoires:
You will need in most cases angular (=gewinkelte) connectors if you want to use the Aquatube as reservoir. Just read through
Dinos Foto Story to get to know how to install an AC watercooling system.
And I recommend to buy a T-Stück and the plug (Shop -> "Zubehör" -> "Plug&Cool") for easier emptying of your system. Just install like this:
You can open the reservoir and the plug to do so.
And last but not least order some angular connectors (=Winkelverbinder PnC) and about 3m PUR hose.
So, I wrote a whole novel and hope that your questions were all covered...