It was my Grandfather's but he passed away in November

He was the second owner I believe, and from memory they purchased it at only a few months old

1985 Rover SD1 Vanden Plas 3.5L V8
It's currently sitting in the shed slowly rotting away

We've had a few people interested in it, but no one actually buying it, the best offer so far is $500.....
The Cons:
Some rust, not sure how much though
Leaking Gear box, goes through about a L/Week
Hasn't been started in about 4 Months, as it now has a flat battery.
I'm considering purchasing it off of my Grandmother, but i'm not too sure what to do with it.
Whether to make it a rally car, and chuck a supra 5Spd Gearbox in it maybe...
Or to restore it to fully working daily original condition....
Thoughts?