4bt is too tall to fit, you'll be bashing the oil pan into the diff if I recall correctly. There is an Isuzu diesel engine that works really well in them, although I've forgotten which engine. I've seen a few D2's with those. The transmission and transfer box on them are PRETTY stout, so I wouldn't really worry about them too much beyond regular PM. Rover engines are just a little hit and miss in my opinion. Some people put 250k miles on them and never have a problem, some people blow head gaskets every 50k. I had innumerable troubles with my 4.0 in my D2. Then there are other folks who never have a single issue. So, are they reliable? Yes. Are they all reliable? No.
I've seen enough of them that have lasted forever to have faith that they can be a reliable platform. But they require more PM than most.