The TD (simple turbo diesel) was rubbish. Avoid like the plague as any still running WILL have problems. Mine started consuming its own oil and had to be crashed to stop it running! I now have a TDi (TD4) in the same vehicle and it's brilliant. The donor was a Discovery, and it's not a simple conversion, but the engine is faultless. It's not only got more power than the TD, but it starts really well and is considerably less thirsty.
My Defender is a 110 station wagon. It's noisy, rusty bumpy and prone to leaks and condensation, but this year is the second it's sailed through its MOT, and I love it to bits.
Really vintage Landys in good nick attract premium prices. Expect to pay around 5k for something worth having/doing up.
Rust: Take a ball pein hammer and overalls when you go looking and hit the chassis anywhere you are worried - back crossmembers and aft of the bulkhead generally are the problem areas. The front usually has too much oil spilled on it to rust badly. Bulkhead rust can be a pain - awkward to deal with. Doors: always rust out, but replacements are around £300 with the necessary fittings transferred from the old one. You can get almost all parts you need. Middle-row doors for station wagons are like hens teeth s/h, but there are rumours of pattern ones now becoming available. Front pair and back doors are pretty common.
Ex-military are a mixed blessing. They tend to be lower spec, and you really can't tell how they've been used.
Also watch for: gearbox output shaft issues (should be cross-drilled for reliability); middle crossmember on CSWs, Outriggers (chassis bits roughly level with the bulkhead), and power steering boxes.
The wiring is crap, but easy to fix. The heater doesn't (usually). The headlamps wouldn't frighten a gloworm.
Insurance from NFU mutual, usually, or a few other specialists. If you're not planning to off-road it, and it's normal spec. and you don't have a queue of teenagers wanting to learn in it, it shouldn't be expensive, but sadly is no longer dirt cheap either.
The as-fitted tyres (175x14? ) are getting hard to find, but there are slightly wider equivalents around. No longer £25 each, as they were five or so years ago.
Most should have a Dixon-Bate adjustable tow hitch. The ex-mil ones sometimes have a hook, but this is easily swapped. The main DB hitch is expensive as an add-on, but really useful - worth looking for. Also a proper full length roof rack - again pricey to fit, but not too much extra cost on a s/h vehicle. I can put 8x4 sheets of ply into mine BUT you have to fix stuff down at the front really well, as the slab windscreen causes a huge updraft (DAMHIK!).
Most problems you can fix yourself.
They are the greatest vehicle design, probably ever.*
Go for it.
E.
*apart from, er, lots of things, all of which I can't afford, and really only want to look at, not own.
PS: 300TDi engines have quite a few generic issues (top end?), that 200 TDi ones don't.