Second hand without a seconds hesitation...for that money it's by far your best bet.
If stock, i'd say something like an Echo Control, Zoo pitbull , T-rex etc...just something that's low-mid BB and a reasonable length. You should be looking for a rear Pro2 free-hub, or a good fron freewheel set-up. All major parts should be aluminium, and most branded.
If you're considering going mod then I'd say straight away to look at an Onza t-pro. Loved by most people on this forum for their low weight, good strength and great flicky yet controllable geometry. This would be easy to pick-up a fairly decent spec for the £250 mark, certainly far easier than a stock.