How bad is the rust? got any pics? Mokes are pretty rare these days and if it's looked after it'll probably go up in value rather than down. All really depends how much time you've got and how good you are at welding, if the floors good your probably on to a winner.
How easy is it to get spares where you are? as you'll probably have to replace the sub frames, not hard or expensive but a good weekends work for eack of them.
It might be worth talking to a local club and get one of them to check it out.
Run the engine,
Does it start form cold?
(place your hand on it before hand to see it it is actually cold, if it's warm it means that the seller has run it before hand so it starts easilly when your looking at it)
How smooth is it a tick over?
Does the engine vibrate much?
If it's go carpets take them out and have a good look underneith.
Check the sills, A pannels & bottom of the doors for rust
Does it look lowered at the front? If so the suspension cones are shot.
Try and moved the rear wheels in an out, if they move your likely to need new radious arms.
There shopuld be 12 grease points look at them to see if they been regularly greased.
Lastly have a look at www.minimania.com for more info.