Hey mate. Is that site a PC hardware shop or a place that allows you to configure your PC before you buy it off them, sorry I can't read Dutch
By "build yourself" I mean buy the components of a PC hardware store and then assemble the PC, it should end up pretty cheap. You can just read a guide on building and you should then have a pretty good idea, the part which requires the most knowledge is the choosing of parts but we can help you with that
Yes the PSU is most likely upgradeable but still it doesn't change anything.
I'm not to sure of a good PC hardware shop in Europe but someone else may a have a good one they can share.