Shell with ID is the easy thing to find in relative terms, its all the other parts that you will be outbid on every time by an existing owner wanting spares or going back to standard.

Your competing with wealthy people for all the parts you will need, no RS500 parts bargains on ebay any more

Circa 4 to 5k for the ID though, original shell is just a bonus.

Better off building it up as a top notch high spec car rather than aiming for the big bucks standard car values that everyone sees.

Still cost you more in the end though than buying an existing one like Ash has for sale above.