When you put the vehicle in park its the auto box locking pawl that is stopping the vehicle from moving,the electric handbrake playing up is quite common on the Disco 3's and I would advise that you get it sorted asap as it could lock the rear wheels if its malfunctioning which is obviously...
Milage covered isn't as important as how well its been looked after,a low milage uncared for Disco could be in much worse nick than a well maintained high miler
Check the height sensors are still attached top and bottom as a pothole could have popped a link
Failing that find someone with hawkeye or nanocom and check for fault codes
Sunroofs or screen top corners,mine used to **** water in mainly from the nearside top corner of screen,it would then drip down from the interior mirror and kill stereo head units.
Depends on how its been cared for,with regular oil changes using good quality oil it could have loads of life left,but of course it could be fooked as well !
Aye,definitely get the screen checked out,I never did find out exactly where mine let water in as I sealed right across the top and down both sides,but it did cure it.
Replied to your pm btw;)