Do it yourself bro. get some practice in and you'll be set. http://www.casemodgod.com/index.htm has a tutorial i think
buy 200 grit automotive sand paper, sand your case to bare metal. take 500grit and then 1000 grit and polish it smooth(you cna use slightly different numbers those are just guide lines) then buy automotive paint and spray. for the best results disassemble your case as much as possible. it might require grinding through a few spots and using your own bolts to replace the former holding mechanisms. only advice I can give since I'm not in the UK.
+ to add on xlink's point. Remember, 5/6 light coats looks 100x better than 1/2 heavy coats. Also, prime it, and clear coat it.
Here's a uk based website, but it looks like you have to buy a case before you can have it painted and you can't already use your old case. http://www.kustompcs.co.uk/index.php
Kustom is one of my favorite sites for casemodding but if your planning on doing it yourself follow all instructions from xlink and use a proper airbrush sprayer, not the heavy can sprayer, will just end up gloopy and drippy.
Not if you spend time and do many light coats it will come out ok. Also, best to do it somewhere like room temp, so not the garage. + an airbrush kit is costly if you wanta decent one, plus you still need a compressor