Well, I did exactly as you are considering. In Sept of '02 traded my 99 Ford Explorer Sport for my current 96 D1. My Explorer was a 2 door and 2wd w/ 59K mi and I wanted a 4 dr SUV with solid axels and excellent off road capabilities. So that left me with the typical 3 options: Jeep, Toyota Land Cruiser, and Land Rovers. If you really want the LR, my advice is (if you want it):
1. As some one else said, buy as new a LR (in as best condition) as you can afford with the lowest miles
2. Bonus points if you can get detailed records detailing excellent care from previous owner
3. Get a certified truck if possible
4. Definitely get an extended warranty - so you may have to buy from a dealer.
If you can only afford a disco with around 100k miles look for the following repairs/needed repairs (this is what I have done)
1. oil leaks, especially rear main oil seal (or any other oil seals for that matter)
2. suspension (shocks, bushings and springs) shocks are usually replaced around 70k miles
3. Ancillary engine components (alternator, starter, water pump, etc.)
4. Radiator repairs (ie: rodded)
5. Steering box leaking replaced or is leaking
6. Exhaust system (at about 100k cats fall apart and clog up the exhaust, and therefore you lose performance)
7. Any signs of rust/corrosion
8. The over all general condition of the truck, both inside and out. You can tell if the previous owner kept it clean and in good repair or not. If it's in good condition aesthetically chances are good that it probably has been well maintained mechanically too.
You might also check out robisonservice.com and look under the service departments/land rovers/buying advice section. Some good info there too. And hey, while you're at it...call a LR mechanic and ask their opinion.
I wish I had known all of this when I bought my truck. I have spent enough money repairing my 96 (now with 108k miles) that I could have bought a 2nd disco just like mine. It's a love hate relationship. I haven't had to repair it lately so right now I love it. Seriously though, considering all of the faults mine possesses - I do enjoy it though it has been much more expensive to maintain. But it is that much more capable than the Explorer.
These are just my personal opinions and experiences