If you search around you could pick up an 2.5 swb for as little as £500 with MOT and fully working.
@£100 thats cheap as a car for spares, it could just be a new gasket it needs, or it could be a new cooler, torque converter, gearbox , transfer box.
Then you have to start sourcing parts, new would be out of the question, are you competent enough to find the fault and fix it?
Unless you can find where the leak is coming from, before you buy, i think i would steer well clear.