The only problem you'll have with this plan physically is that ISO has the extra key next to left shift, an ANSI set wont allow for this. Another thing to think about is the legends on the keys other than the main letters which may be completely or partially different to what the key types. Wikipedia shows French and Dutch "Belgian" keyboard layouts so that doesn't help wiith what you're used to buy have you looked on
PMK to see if any of the international kits will work for you? (link is pre-filtered) Not cheap and you may have to pay import fees too but they are top quality (PMK is the website of Signature Plastics who you've probably seen mentioned in group buys...)
As to the advantages of ANSI - what do you see other than lots of pretty keycaps and it being supported by every board? If you're going to build a programmable board you can do better fitting things to your use like putting backspace on Caps Lock (which I stole from the Colemak layout) or if you use Ctrl more put that there instead (as seen on the HHKB) ANSI enter absolutely broke me when I tried to use a HHKB as I quickly learnt to hit the key above enter for backspace but still hit the same place for enter as I couldn't move my muscle memory from hitting the top of an ISO enter key. Also if you learn ANSI and try to use someone else's computer and everyone else has an ISO that could be a problem.
There is a lot to think about so well done for not just buying random stuff and hoping it all works together
