I'm 6ft and have had a 15.5" and 17.5", I'd say if you're going to treat it as it should be treated (read thrashed/crashed/jumped/DHed and razzed) go for the 15.5", if you're going to use it as a xc/trail bike, go 17.5". I have the 17.5" as my only bike now and this works as I need to rise up down and around, if I was using it as more of just a play bike i'd still go 15.5" despite being 6ft.
I'd say 15.5" for yourself. Oh and I paid £130 for a 2004 17.5" one second hand, shabby paint.
£250 for a second hand 2007 one (in 2008) with evolve XC post and hope seatclamp.
I'd say for a good condition one, you're looking at about £225.