Hi ed,
Don't forget also that with no tax, you can't drive it back from where it is, so budget for trailer hire, £60-100 depending on how long you borrow it for.
As said, could have any amount of problems. I don't know anything about the 2.25 diesel so you're guess is as good as mine with the engine. Springs n shocks (and u bolts) are cheap from Paddocks, around £20 for springs. Don't forget tyre conditions, something quite often overlooked, it all costs money.
I'd be wary of the brakes and probably clutch, possibly gearbox, how long has it been sat there? Always test drive and ensure high/low/4wd all work, though with no tax or test, you can't really (not legally anyway).
Series IIIs come and go, so unless you plan to do it up and sell, and you can get it for £200, £300 tops, I would leave it. If you're going for it to keep, make damn sure there's not much wrong with it, Series IIIs are plentiful, and 1976 is an OLD Series III, you could get one sub £1000 that can be up to 1984.
Did you get the reg?
If so visit this site, type in the reg and it will effectively tell you when last taxed.
DVLA Vehicle Licensing - click Vehicle Enquiry
RACRegards,
Rob.