This type of question comes up fairly regular on the forum, and there might be a few good solutions obtained by checking the archives.
I would make a suggestion that you perhaps look at bare-bone systems, where the components are supplied as a compatible bundle. This would save a number of problems, if its a first build. Have a look at somewhere like Novatech as a start. There are a number of other suppliers who would have bare-bone systems on offer for comparisons.
Once you have an idea,that might suit your pocket and expertise, then come back with any questions.
Would mention that for £450/£500 you could get something off the shelve with a manufacturer's warranty on the complete unit. If you go this route, then check if Windows or Linux is included in the price. Ebuyer might have some special offers here.