med/large hardtail wanted

a 17 or 18 inch hardtail ...i think kid's about 5 ft 3
-gotta be able to use the bike straight away -i'm not buying bits of bike in pieces here n there
is it still too early to be looking out for a 2nd hand bike w 29inch wheels?
kid's out on the bike a lot so great shocks is a must- he'll need to be able to try the bike out for a few minutes somewhere mucky n rocky rather than just round a housing estate. dervla
that's a nifty looking bike Ian-my car broke down yesterday so i have to be reeally tight w. money! good luck w selling the bike tho.