I cannot answer all your questions,also there is a flipant and answer that all starts to fall off from new, but suggest you take the advice of AAA or similar.
You do not mention offroad, but if you do intend to enjoy the disco in the full then probably look to get an 04 with the Diff Lock as as an option, generally not available on earlier models , not essential but would be a nice to have.
Mine has 100k on the clock (2000 my) and no leaks or major mechanical issues. You will already know but get one with a good service history, ideally with AAA or similar, and as you mention get it checked before you buy whatever the cost it is worth it