Wow, that sounds like a serious project!!!!!!
Sometimes what something is worth can also be worked out by how much money it needs to fix.
A resto will never be worth original, an saying parts are worth eBay dollar don't mean they will, especially if they are
Damaged due to the elements (rust)
£800 doesn't seem wrong for the car although I wouldn't want to rebuild a car 'that bad' myself.
Just make sure it's not sentiment of seeing the car often and slowly rot away that's not fueling your buying.
Then again mate, you will do what you feel is right for you.
If you don't get it, you'll find one better an if you do good luck and I would love to see another build in projects an another osf come from the grasps of the crusher ykt