Build your own will get you the PC you want (assuming you spec it right), but it will probably cost you a fair bit more.
I used to build my own, about every three years, and reckoned on £1,200 for the spec I wanted. I think I always overspecified, and didn't use most of the features I built in.
I bought a cheapo off eBay two years ago, for £500. It was about £200 cheaper than I could have built a similar spec machine for. I've had to replace the graphics card (fan on it failed, it overheated and died) and the CPU fan (a large moth got in the case, I know not how, and jammed itself in the fan). Those two bits cost me under £100.
Apart from that (and Microsoft software glitches), it's been fine.
I don't know how these eBay sellers knock them out for the prices they do.