In the last two years the price for mint standard Escort Cosworths has gone up not down.
£20K today will buy you what £17K would've bought two years ago.
Every year there are less and less mint, standard, low mileage, original cars, it's simple supply and demand and a sellers market.
Buy a mint, original, standard, low mileage Escort Cosworth, don't put too many miles on it and you will never lose money on it.