I've got a few knocking about in various states of undress. I'm currently swapping the chassis of my Series 3 109" truck cab (pickup) for a galvanised one. Also have a SIIa 88" which is waiting for the same attention.
Values seem to be climbing steadily so I think they're one of the only classics that you can restore without exceeding the value of the finished vehicle.
As Harvey says, they rust very nicely. But you only need to worry about the chassis and bulkhead, in that order.
Buying one, I wouldn't pay more than 800 pounds for one with its original non-galvanised chassis no matter how good it claims to be. Better to pay less and replace the chassis yourself or pay over a couple of grand for one that's already been done.
An 88" galvo chassis costs £1200 or so.
A good bulkhead will be £300-£500, more for a SII or IIa.
Overdrive is a must. They currently sell for £300+ used.
Nothing else matters. It's just Meccano and most parts are very cheap, though rebuilding a front axle with new swivels can mount up to over £300 (but there's no shortage of S/H ones).
Engines - petrols are kinder on the eardrums but not the wallet. Decent used ones are barely scrap value.
Lots of folk are now fitting 200 or 300TDi diesels from Discoveries and the like. Good economy, but noisy.
But if you haven't driven one for 10 years, I'd suggest trying one again to remind yourself how horrid they are. If you're over 6ft they'll give you back-ache - I can't drive mine for more than 90mins at a time without suffering for weeks.
Steering is vague and very heavy.
Ride is appalling.
Noise is unbearable.
You get wet when it rains, even under a hard-top, and cold.
Zero security so you can't leave anything valuable in them.
Slow.
Think of them as tractors with extra seats and you won't go far wrong.