#1 With GTX 660 Ti/670 you will be able to max all games of today and most games of next few years. If you can afford GTX 670, it will last you longer due to wider GPU bus width. Thing is, when buying a graphics card you should always try to get the best one you can, because it will mean you'll have better performance longer and it will delay the time you have to upgrade again. Although i've seen many people recommend either getting a 660ti OC'ed, most overclocks allow it to reach roughly the 670's performance or get the 660ti then in the future get another and run in SLI. Final decision is yours.
#2 I often use overclockers.co.uk (Not sure if will ship to Belgium probs will) But, if not i just use Amazon and eBay to be honest. I've got amazon prime so thats a plus as well as i can often find multiple sellers for one product if it happens to be out of stock. Just make sure you're careful with who you buy from on eBay, just to avoid problems in the future
#3 Yes, that's what i done on my first build to save money.
I'd be happy to answer any other questions/problems you may have