You only need to worry about rust on the chassis really - and only if it's been knocked about. The chassis is huge and tough but it is is steel. Most of the body work is alu (another thing in common with my Mog) so little to rust there. I think the 90s have a bulkhead (?) behind the front seats which limits how far back the front seats can tilt and slide - the 110s (Station Wagons at least) don't have this.
The rear seats are good enough for squaddies so should be good enough for the rest of us! Get the fold up ones like in your first link - I've got benches in the back which don't fold up and sometime get in the way of stuff you want to chuck in.
If you're gonna use it for hauling junk around get a dog guard to go behind the front seats so nothing comes hurtling through when you stop suddenly and the whole vehicle pitches forward!
300Tdi engine is tough, reliable and home servicable (I'm lazy and don't though) - TD5 has more electronic gizmos but has proved to be better than people expected.
I suggest you look at condition rather than age - spares are pretty cheap and you can get just about anything for any age and they're easy to fix.
Look for mud in the nooks and crannies of the chassis and piled up in the coil springs as signs of off-roading - you don't want to get one that's been knocked about unless you plan to knock it about some more then fix it as as hobby.
Good luck - and be prepared to wave at other LR owners in Defenders, Series, and tricked up Disco's, RRs and Freelanders just as enthusiastically as you'd wave to another Mog owner - but be prepared to wave more frequently.
