200 series - very simple, barely no electronics, just electrics. Gets the job done, impossible to not repair. Not the comfiest/well refined but certainly won't stop without a fight.
300 - the onset of electronics. You got window ECU, immobilisers, lambda sensors etc etc. Albeit a much better car, and more pleasant car to drive on and off road but you may find you get it wet and something won't work without rhyme or reason. Still easy to repair and bits are cheap though.
D2 - probs alround best disco ever. ALL ROUND. it's great on and off road, much more sensible road manners and very good offroad out of the box with traction control. But, it has it's failings, and these are usually electronic. A D2 may leave you scratching your head or comforting your wallet for quite some time. Worth mentioning that it won't be rotten (generally) and it probably won't leak water in when wading.
Overall:
Older the better for predominantly off road, a 200 can be soaked day in day out and all it will need is a starter and alternator eventually.
Conversely, a D2 can hammer motorways all year round and be easy to drive, but it might mysteriously break down one day and cost you a few hundred.
A 300 is in the middle so can do a bit of both admirably.
Don't expect one to do the other ones job, and you've found the right car.
