Hi UFO, my advice would be to buy the best, mot'd donor you can afford and start from there. Just make sure you do a proper inspection of the chassis and front bulkhead/footwells.
I paid £845 (ebay) for a donor that was described as practically mot ready, just needing a driver's side kick plate and 'a bit of welding in the footwells'.
It turned out to have a rotten chassis that someone had disguised by wrapping tin plate round the rust and coating it with underseal, while the front footwells were worthy of Fred Flintstone's daily driver.
That last pic is what had been tacked over the rust in the chassis main rail where the o/s front outrigger connects
Luckily for me the body panels were in much better condition and I actually made a profit of about £30 by selling them all off on ebay for around £870.
Restoring and preparing the chassis and front bulkhead took me half my build time - a year just to get to the stage where it was a suitable base to bond the body on - so I could have finished the car a year earlier if I'd started with a decent donor in the first place.
The end result was worth every bit of the time and effort for me, but I'd definitely go for a £2000 car as a base if I did another and thoroughly inspect the chassis and bulkhead prior to purchase.
Good luck!