Lets be honest, they're all a bit of a lottery as we're talking about very old cars now.
Disco TD5- getting difficult to find one that isn't terminally rusty underneath; the last few I viewed looked perfect on top but were getting very weak in the chassis
Also, I'm not convinced the TD5 is overly reliable, fuel pumps, fuel leaks, injector washers, injector looms, head problems, oil pump bolts coming loose etc etc.
P38 Diesel - feels slow unless chipped, questionable reliability of the BMW engine, some have major issues, some don't. Body and chassis VERY good at holding back rust. Usual P38 electrical problems.. there's a lot of very thin, brittle, corroding wires under your arse
P38 V8 - as above but with one major caveat. The V8 engine can slip its liners and grind to a halt in a cloud of steam. Sounds unlikely I know, but this is a serious problem/ inherent flaw and will easily write-off the nicest example unless you're happy to have a £4k rebuild done.
Sounds a bit gloomy (sorry), but the reality is, nothing will be as simple and reliable as a 200 or 300TDi. Having said that, if a P38 4.6 HSE, with LPG and a Turner Engineering rebuild came my way, I wouldn't be able to resist