Welkom op het forum Panja!
I'm from Belgium and I'm just finishing up my own first build I started this summer, so I know exactly what you're going through
. First of all, let me reassure you, you've come to the right place. This community is populated by the best experts around on anything arcade-related. You can not believe with how many big and small problems these guys have helped me (and many others)!
As for the arcade shops, you needn't worry about where they're located. For my first build, I started buying stuff from ebay and arcadeshops around the world. No matter where the seller is located, as long as he's willing to ship to the Netherlands (they most likely will be), you'll get the products. The only thing to consider is shipping cost and deliverytime. Almost all sellers have great customer support, and will gladly resend your package or refund you IF it gets lost in transit (my joysticks got lost in transit and I got new ones right away with some extra balltops for the inconvenience!). So no need to worry there, just look around for what you want and what seller is cheapest. On all matters, the BYOAC wiki is a great source for information. It also has a relatively complete list with arcaderelated vendors with their location and some information. Check it out!
http://wiki.arcadecontrols.com/wiki/VendorsThen, wiring and Ipac. First of all, Andy from Ultimarc notes in the official Ipac "literature" that any gauge wire will suffice as long as it's thick enough to be clamped by the Ipac connector. So go with whatever you have laying around. On a little sidenote, as Jipp said, don't use wire that is too stiff/thick. It's a PITA
I got my Ipac4 with PS2 cable, and I have no complaints about it whatsoever. It's supersupereasy to setup (even my mother could've got that thing to work...) and I absolutely see no reason whatsoever to pick USB over PS2. Ofc I don't have experience with the USB setup, but I can warmly suggest the PS2 version.
As for the 70cm TV, it's your cab, so you can make it into whatever you want. I do however think 70cm is a bit of overkill for a regular standup cabinet... I used 50cm, and that's just right. Also, a 50cm CRT would allow you to just use it as a whole instead of having to decase it. I decased mine, but if I were to do it over again, I would DEFINITELY leave it cased. But again, it's personal taste...
Best of luck with it, and if you have any more questions, shoot!