I wouldn't be frightened off by any of that. Just make sure you give it all a good check over and verify the ID with the DVLA to be sure it's legit. As Mike says, the fact it has been looked at and Q plated is a good thing. Regarding the mileage discrepancy, speedo heads are so easily changed it really doesn't surprise me if the truck has had loads of other work done too. On an old Defender mileage is largely irrelevant anyway, you are far better off inspecting the condition of what you can see in front of you. Take a seasoned LR mechanic along to look at it, they should be able to spot any tell tale signs of things that are about to implode.