If you go for a D3 you will get more refinement and usable speed on the road. Go for one that has a full service history if possible, they are complicated beasts, and not much or no offroad use (although obviously they are excellent off the road).
If you go for a D2, offroad ability is still a given and although it'll keep up with traffic, they can be a bit more tiresome on motorway journeys, if that's an issue. To be fair I've driven up from England to the Orkneys in one without missing a beat, but the D3 would get you there with less stress. To minimise problems I'd look for the latest D2 you can find, the facelift ones really (02-04), look for one without fitted showers (some call them sunroofs), normal roll bars, not the ACE and springs on the back rather than air suspension. That lot will wipe out a good degree of potential problems and bills. When looking, get down and dirty at the back end and look at the rear sub frame. If rust is an issue, that'll most likely be where it's at and is the cause of many a D2's demise.