Edit: Just noticed you are from the U.K., so removed U.S. vendor from post.
Fantastic vehicles. Before I make some comments, I just want to write that D2s are not that hard to work on and can be maintained on a modest budget if the truck you start with is a good example and you are willing to work on it yourself. Who knows, maybe you can afford a shop to do the work? If so, find a shop that specializes in Rovers, as they are very much different from other vehicles and can burn out mechanics that don't know their common issues.
The V8 Rover used in these trucks has a reputation for being "charming," to put it kindly. Expanding on the above comments, these engines are an absolute nightmare if previous owners skimped on maintenance or used cheap fixes. Even when meticulously maintained, they are temperamental and need constant attention.
Though many D2 owners will have their own suggestions about what perils you may face, look out for:
-overheating. Check that the coolant level is correct on the cold engine as a first check on a D2. Take the cap off the overflow reservoir and look for evidence of stop leak. I would run, not walk, away from a D2 with cooling issues. Unless you are a glutton for punishment or just plan to potentially swap engines, cooling issues cripple these trucks and send them to the junkyard on a regular basis. This could be anything. Really. The engine blocks are notorious for cracking behind the cylinder liners and can be very difficult to diagnose. Typical presentation will be the truck runs fine until under load, then starts overheating shortly after putting in gear and starting on your journey. If you get a D2, people recommend monitoring the coolant temp via scangauge/ultragauge/OBD2 Bluetooth adaptor and Torque app on smartphone, etc. This is because the temp gauge stays in the middle until the engine is already overheated to the point of serious damage. Only once you have cooked it will the needle budge. I got a bluetooth adaptor for $20 and an android tablet for $30. Cheap insurance.
-ticking noises that could be anything from a lazy lifter to a cylinder hammering away the head gasket fire ring. The former isn't really a worry, but the latter issue will eventually require engine replacement or repair with flanged liners. Flanged liners are favorably discussed at length all over the web and are viewed as a panacea for what ails these engines by some. I used flanged liners in one of my D2s and can't complain. A machinist will have to install them, though, which costs about $2k in my case, plus the $600 or so for the liners themselves.
-3 amigos (3 warning lights for traction control, ABS, and hill descent control that all illuminate together signaling entry into the elite D2 owner's club). This could be a wheel bearing, an ABS shuttle-valve failure, or a slew of other issues. An Autozone code scanner will not tell you what it is or allow you to read or clear codes. If you get a D2 and do your own work, there are scanners you can buy for modest prices to read and scan these codes. The alternative of throwing parts at the truck can run in the thousands of dollars if using genuine parts.
-electronics that don't work (specifically, the door lock actuators fail often). Like all cars, electronics fail.
-and vibrations when driving. If you get a D2, check the front driveshaft. If it has non-greaseable u-joints, swap in a shaft that is greaseable and grease it every oil change. A common failure mode on these trucks is the front driveshaft fails without warning on the highway and smashes expensive holes in the transmission and passenger's side catalytic convertor.
None of the things that go wrong with a D2 have to be expensive if you are willing to get your hands dirty. Every problem you will likely encounter has been discussed many, many times in forums and there is still an active community of people that are chuffed to bits to own these trucks. I own 2. Best vehicle for comfort and capability for less than $50k, if you ask me. Just be sure to go in with your eyes wide, wide, wide open.
A final note on servicing D2s. Owning a D2 and feeding it quality parts will save you lots of heartache. My experience, and the experience of many frustrated people posting in forums, usually underscores the intolerance for poor-quality parts on a D2. Buy quality parts to service a well-maintained truck (with records from new, if possible) and you might just fall in love with it.
Good luck and happy Rovering!