First thing you have to remember and prepare yourself for is that it is a Rover and more importantly British. They will require a bit more upkeep and maintenence then say a Toyota of the same vintage. This is why well documented maintenence is important. Often times the little problems don't get fixed which later become bigger and more costly problems. Used parts are abundant and if you're a DIY'er this might be the truck for you. If all pans out, Rovers are tanks that will never let you down.
Off hand this is what would check:
See if the swivel balls are leaking at all on the front axle. If so the inners may have a blown seal or the whole thing will need to be replaced. Can be costly new, can also be used as a bargaining chip.
Dual sunroofs are awesome if you can use them. Disco dual sunroofs are notorious for being crap. They become misaligned over time and either stick when you want to open them or don't open at all. If they stick you can't really wheel with them open because they will make a ton of clattering noise that will make you think the glass is going to shatter over your head. Parts are easy to come by used but components pertaining to the track mechanism are not sold seperately from the dealer. If you can live without them working properly or at all, cool.
Check the window rubber around the rear glass and alpine windows. If it is cracked, as they usually are, plan on replacing them. You can get the rubber for a good price from RoverConnection and it doesn't cost a lot in labor. Now if you live in a climate that is not arid, look for signs corrosion, especially around the alpine windows.
Is there oil seeping from the headgasket and valley pan gasket. If so, you will have to change that eventually.
Valves on high mile Rovers are notorious for sticking, especially when used with anything less than premium gasoline.
Does it overheat. Check the fan clutch. Check to see if there is any seeping around the water pump.
The power steering will likely be leaking. See where it is usually parked to get an idea of how much it leaks. If they haven't steam cleaned to motor, see how wet it is near the reservoir, box, and undercarriage.
If it hasn't already been changed, the oil pan gasket will be either seeping or leaking. Check it out, make the call.
Does the T-case leak?
How bad does it leak out of the rear main seal and t-case seal.
Does the Center Diff Lock work properly? If it won't go into low, the hi-low solenoid switch will probably need to be removed or replaced, it'll work after that either way.
Does the cruise control work. If not you will need to change the rubber vacuum lines because they are likely cracked.
Do the windows roll up and down easily. If there is any creaking or popping sound, the nylon wheel in the window regulator is going to pop and break in half. Once it does that the windows will no longer stay up. You can buy the parts new or used (but not the wheel), or fix it yourself with nylon washers and parts readily available from a hardware store.
Does it have exhaust leaks or a cracked manifold. It is a common problem with these things. Gaskets and hardware will run you about $100 excluding the manifold if needed.
Is the belt tensioner in good order. They go bad and often go un-noticed.
How do the brakes feel. Rovers aren't known for their performance braking but they shouldn't be too spongy.
Has the electrical system been messed with. Often times previous owners like to add a lot of stuff to their Rovers which can lend itself to half arsed work and the all you can eat spaghetti nightmare buffet.
You are not likely to have all of these problems with this truck, if so RUN because you should be able to find a better truck for the same price. These are just the things I have seen on other higher mileage Rovers that exist. The leaking will be inevitable but the difference will be a flesh wound or a ruptured corroted artery. Take an inventory of what you find to be wrong, put a worst-case-scenario-highest-cost possible list together and see if that number will work for you. If not, walk away and pay a little more for a truck with less problems and less mileage. You may even have to settle for a bone stock mall-crawler, but who says that's a bad thing.