Budget Machine for oclHashcat (noob)

So yes I've seen dozens of posts about which hardware will run oclHashcat the fastest..... but that is the aim for those Ops ... to get fast (and expensive).

I'm looking to start off with a machine that will just get me up and running with gpu Hashcat .. let me play with some small dictionaries, rules & attack methods against known test hashes.
Technique over quantity.
Once I've got the skills, then I'll think about investing the £s.

So I'm likely looking at just getting a 2nd hand tower, mediocre CPU and best GPU I can.
This machine will pretty much ONLY be used for password cracking.

Budget under £500 ($750).

Questions :
1) How 'mediocre' can the CPU be ?
2) I am understanding that in my price range, I'm best going for a Radeon HD - the highest I can afford ?
3) Cooling. At what stage do I worry about cooling ? If I've got 1 card running for a few hours, do I need to be concerned about 'gaming' cases and fans ? What about 2 cards for a couple of days ? ... I guess if anyone can point to reading matter on this subject, I'd appreciate it - hard to find !
4) ... any other pointers tips or 'pointing out what the idiot noob has completely misunderstood' !


Many Thanks