Like s1 turbos piles of scrap start at 7k nowadays.
Wouldn't touch 1 with badly fitting panels plus other issues if you plan on keeping it as it will be a bottomless pit of money if you try and make it a minter,polishing a turd springs to mind.
Keep an eye on the sierra cosworth fan page on facebook,cars appear on there every few days but dissapear quickly,even the sheds and every day there's someone looking for a 2wd saph.
It's not cheap to source original good condition parts nor is it easy to find a car so don't get impatient and buy something half ok unless it's cheap enough to do something with.
Speaking from experience,I bought a very good saph fir 15k and spent the same plus a lot more on it but once done I'll have a nearly new 1990 saph.
Happy hunting!