Welcome to the madness...
Any Disco will be capable off road and there are plenty out there to pick over...
Spend some time in the Discovery Forum and you will get a feel for the good, bad and the ugly - 300's are rather more refined than the 200's but you really need to ge the best condition one you can find... don't get hung up on age, mileage, reg plate etc.
Rear floor in the boot and under back seats rust out - and when they do, the rest of the car ain't far behind generally. Look for telltale water marks on seats from leaking sunroofs - water in here means it has to go somewhere - normally by going through the floor!
Sills, inner wings, open the rear doors and check inside the door shuts (wheel arch)... not all Disco's will have a wheel arch...
Find your nearest pub meet, wander along and introduce yourself - then get someone to take you round theirs and point out the areas to watch.
Then enjoy....
All the best!