Just have a look and see how straight they are-- i'd imagine they're pretty good. Have a look at the grain too and hopefully it's quite straight without much runout. And just check there are no nails/screws that are buried in it that might foul up your tools (or cause an accident if it catches on a planer etc.).
I think they'd probably be quite stable and you might get away with a single piece neck, although you could rip it lengthwise and make a 3piece to be sure it wouldn't warp.
Wez, that looks lovely! I've got a few nice bits of wood kicking around, but i've not done any guitar building for a few years now :(
when my parents got pvc windows in, and the 30 year old mahogany frames were ripped out, i kept a couple of those. they're certainly big enough to make necks, any idea how i would know if they're actually any good?