From my so far limited knowledge of the Land Rover vehicles (but I am learning fast!) I'd say that the later the vehicle, the more complex the vehicle becomes, especially with the electrics and electronics.
My Son's Landy Series is so simple, no electronics, mechanically quite bullet-proof and so on.
Get into the Disco 2 and you're into a LOT of electronics, and if that is not your scene then it can be a nightmare. We run an electronics manufacturing company, so most of the stuff is familiar, but to a non electronic person it can be expensive to have to pay for work to be done.
Mechanically they are very similar, with detail differences, the TD5 engine has its problems, but so do them all.
Peter