I'd have thought it practically impossible to spend £2500 on a s/h DH bike. you'd get a great spec V10, Sunday, Glory etc. for about £1200-1500
so even if the frame cracks you've got a spare grand to buy another.
you could go down to innerleithen next weekend (winter series race) and have a look around the pits, there's bound to be a few bikes for sale and with cash in yer pocket you may well get a bargain.
BTW – an M6 and V10 may have similar suspension designs but ride quite differently. (coming from Coves for instance, I doubt you'd get along spectacularly with the M6s BB height) what bikes have you ridden? and why did you choose those? I don't how you ride, but I'd be looking at geometry before prices?