What Grant said - joining a good club will pay dividends. Having a poke round the forums here will give you insights into ownership - what goes wrong, how to fix it and stuff you can tweak.
An older rebuilt vehicle with a galvanised chassis and a good bulkhead could be a better bet than just buying the newest you can afford, condition is more important than mileage.
A truck covered in accessories is never worth the sum of its parts so don't fall for anyone who tries to make out that a £2k vehicle with £3k worth of extras is worth more than about £3000, and that's if it's tastefully done, well-fitted and quality stuff that you actually want.
Chequerplate is worth precisely nothing and goes down in value the more of it there is plastered over the thing, especially when it's used to hide crusty panels, dents, and rusty bits of chassis.
As for what to look for:
- Chassis rust, have a damn good poke and be suspicious of stuff caked with waxoyl/black sealant, it should ring true not rattle or thunk (or fall apart) when tapped with a knocking stick.
- Bulkhead rust, see above but be on the lookout for filler and stuff hiding behind hinges/panels and bubbling out.
Everything else is varying degrees of nuts and bolts. If it's a 200/300 TDi check it's good as a new one is money. Although most things can be fixed, TDi heads can still get expensive.
A few oil leaks are usual but nothing should be dripping wet or need constant topping up. The power steering box will always leak from the bottom seal, a little moist patch is probably to be expected, more than that and expect to have to fork out for a new box at ~£200.
Door frames and body cappings can rust, doors are money (£250 pair) and can get beyond repair without showing it past the plastic trim. Cappings are a fiddle to replace but not rocket science and early galvanised ones stop the problem.
Gearbox should shift nicely and not be noisy.
Transfer box (hi/low & difflock lever) may be stiff to change if it's not been used off-road, they are pretty solid. Only weak spot is input gear which will manifest itself as a clonk on taking up drive and going from forward to reverse. Assuming the gearbox splines aren't too worn it's £50 for a cross-drilled input gear and ten minutes crawling round underneath to change it.