Would agree with PotGuy. Too risky.
Your best bet if you're not confident of building it yourself is to find a local computer shop,somebody who'll build you a system for a reasonable fee, a dealer who's been trading there at least five years (see Yellow Pages).
Then ask here for advice, many people will spec a system for you.
Present that system to local dealer, ask 'How much' then negotiate/bargain/change spec to suit budget.
Or find somebody on this forum who's local to you, they'll build it for you. How much they'd charge, I'm not sure, I'd guess around fifty quid to 100 quid, depending on how much software you want loading on/updating/patching.
That is just a guess though, I don't actually build systems myself, unless it's for close friends.