any pointers to anything I should look for in terms of wear and breakage
I got my M1A in 1977, shot it a lot, I mean A LOT. I've worn out barrels (shot out) and I've stretched out some slings.
Other then that I haven't broke anything. It's hard to tell about the barrel with out shooting it. I will tell you 99% of the M1As out there haven't been shot enough to wear out the barrels.
The one's that have are guns owned by High Power shooters, but they normally are matched out (Including mine, the NG converted it to a Super Match). You can tell but that's not saying there is anything wrong with one that has been shot a lot.
Easy way to tell, make sure the gun is empty. Then slowly let the bolt go forward, it shouldn't lock up, or go into battery when you turn loose the op rod. Yet a light hit with the palm of your hand on the op rod will push it into battery.
Check the rear sight, Run it all the way up, then push down firmly on the sight to see if it slides back down easily. If it does, tighten up the sight and try it again. If you can't get it tight enough where you can't push it down, then the sight will have to be replaces.
One should check the pistol to make sure its to specs, also the gas cylinder. (over size pistons don't cost that much).
Even the above checks fail, WHO CARES. Easy and cheap fixes.
I can pretty much tell you, without looking at the gun, if its all there, then its worth $900.
About the only thing I can think of to keep the gun from being worth $900 is if it was stolen, which is highly unlikely because most police departments have a pawn shop details that check for stolen property.
Again unless that rifle has been used by a High Power shooter chances are it hasn't been shot that much. Not that that matters.
If you look at this rifle, its not hard to tell its been shot a time or two, but it still shoots to day a heck of a lot better then I do. It's 35 years old.