Yes, the rear chassis needs a careful inspection, especially the rear cross member. Depending on age, all Disco 2s are now at least 11 years old you might well find some wear around the suspension bushes, nothing too much to worry about as long as you’re pretty OK with some spanners.
The Td5 lump is fairly reliable, the oil problem in the red plug will usually show itself first with lumpy or rough running. The main trouble when inspecting is that you have to pull the ECU out to reach the plug which connects to the bottom of the unit.
As I said, expect about 11 year’s use, that could be close to 110,000 miles or so which might well show in the condition of the interior. Check that everything electrical works properly, in some cases with the Disco 2 electrical faults can mean problems with various ECU’s.
Apart from all that, the Disco 2 is generally a pretty good vehicle; enjoy it. On a website like this one it’s the nature of the beast that you’ll hear more about the problems than what’s working properly; there aren’t many postings saying “My Land Rover is running absolutely fine, what can I do about it?”