You should change the plastic 'buds' on the drive shafts, the diff-outdrives should be OK if the previous owner had been a good boy changing the buds when needed.
Check the crownwheels (the main drive gear on the diff's) - replace these if the teeth look worn.
Like any car there is always the main diff rings & thrust race, and of course any bearings on the car.
You need to think of the above as items that will need changing when needed throughout the life of the car, other than those it should just be clean and re-assemble.
The X11 is a good strong bus.
The servo mounts are just an upgrade item I think - I've only ever run the standard plastic ones with no prpblem.
Mods - depends on what has already been done and what you want to do, mainly they are electrics layout changes and suspension mods. There is loads of stuff in previous threads on this forum and on
www.the-pred.co.uk, far too much to put in one post