I shall probably go with a ready-made computer rather than building it from parts; I think that the extra peace of mind of knowing that I can send it to someone for repairs rather than have to figure out what’s wrong and replace the part myself will be worth the extra money I pay for it.
I’ve tentatively settled on Grey Computer Cologne: they offer quite a wide variety of base PCs that you can modify by adding or replacing components; they have a 5-year warranty on their machines (with free shipping in the first 3 of those); and they say whose manufacturer’s components they include (e.g. Samsung memory) rather than just saying “1 TB HDD” or whatever and then sourcing from the lowest bidder.
I’ve read around a little bit and found some good reviews and some bad ones, but not so many bad ones to put me completely off them.
Another point in their favour is their forum, where people can discuss things (such as “Where is my machine! It’s been four weeks now!”), and they even have forums where volunteers provide configuration suggestions! So you can either say, “I’d like a machine to do X, Y, and Z, and I don’t want to spend more than €N on it” and let someone configure together a sample machine they think will do what you want within your budget, or you could say, “I’m thinking of buying a machine with components X, Y, and Z; does that sound right?” and have them tell you whether your power supply is over-specced or they’d recommend the slightly-more-expensive case because it’s easier to cool or whatever. I asked a question there with the configuration I had in mind and got a couple of suggestions (for example, that 8 GB RAM is probably overkill for my purposes and 4 GB should be just fine, and that I should perhaps instead invest some of the difference in a better graphics card [which, I later found out, tends to be a bit more quiet than the one I had originally had in mind, so that’s good, too!]), which I’ll probably bear in mind should I end up buying from them.
One interesting thing: this computer will probably be about 1000 times “better” than my first PC that my father bought for me when I started my apprenticeship: that one had a 486SX running at 25 MHz, with 8 MB of RAM (at a time when most machines had 4 MB, and I was later glad that I had the extra memory) and 130 MB of HDD (which got upgraded to 2×130 MB fairly quickly). The one I’m looking at has 1 TB HDD (so, about 4000 times as much), 4 GB of RAM (unless I change my mind and do get 8… the parallel of “8 GB at a time when most machines had 4 GB, and I later ended up being grateful I had more” is a bit appealing, though the difference is about €100; 500 or 1000 times as much); and 4×2.6 GHz (so, theoretically about 400 times as much). The biggest difference is in processor frequency, which hasn’t scaled as quickly as the other numbers (HDD and RAM). I don’t remember how much memory my graphics card had back then, but Wikipedia says that the ET4000 had “up to 1MB video RAM”; here, too, I’m looking at a thousand-fold increase, with 1024 MB RAM on an ATI HD 5770.