Are you talking about The kit or the rtr?
Whichever, you need to decide wether you are going for a sidweinder config or a inline config.
Most Prototype cars such as Audi R8c, Bentley EXP8, Lola Bk2/10, Doem Judd, MG lola etc wont go with the sidewinder.
GT cars and tourers will fit the sidewinder, such as a Viper, Corvette, 911, Merc CLK (dtm version) and Aston Martin Vanquish.
Once thats decided:
Scalextric produce the Hornby kits for about £7 depending on if you get one with paints or not. All the boides should fit the sidewinder as long as you make your own interior (simple - peice of card and half driver figure). Scaley interior are just to big for the HRS. These bodies need painting. My 911 got an SRA
livery
If you want something more exciting, As McLaren said, Pendle stock pre-painted bodies of various cars with driver/interior for under £10.
Third option is a resin body from someone like OCA. Ranging in price, the OCA bodies are fairly detailed and come with vacformed windscreen and interior. The bodies will need some work though, its not just spray and go.
Final option is a Revel Easy Kit, Airfix kit and likewise range of static models.
I have 2 HRS in operation. My sidewinder car did have a hornby kit 911 body, but now has a scaley F40 body found from an old broken chassis.
My Inline one has a Bentley
. Carrera donor car.
My final advice is dont buy a donor car, waste of money. Just take your time until you find something you like