You should be able to find a donor way, way under £500.
If you are looking via ebay a simple search for "Ford Sierra" will yeild good results.
Your Location may be a bonus !! I see quite a few suitable donors, way up North. To far to travel for a lot of people & sometimes go quite cheap.
The Sierra parts seem to common on ebay. I doubt the residual value of the remainder of the donor will be worth a great deal.
1.8CVH & 2 litre engines are not worth a lot unless you know someone who is really in need of one.
If you are lucky enough to find a 2 Litre Pinto donor & the engine is good, you can expect to get a decent price for it .
It is still worth trying to sell some of the parts though. Every penny helps.
I paid £100 for my last donor. I sold £130 of parts on Ebay & was paid £50 for the shell at the breakers.
Ps Don't neglect to look in your local free add pages.