Only the Hpi Savage can have a reverse gear fitted as the HPI onroad cars don't and not many onroad cars have one fitted but I do know that the Ho-Bao Hyper One7 Evo RTR Porsche On road car can have one fitted,but the car cost £299 and I'm not sure how much for the reverse Gear but for a first R/C car I would look at all Electric cars as you have forward and reverse and it will costs alot less to run than a nitro car.
All Nitro cars can have a two speed transmission fitted but only two can have a reverse gear fitted too.
Have a look here for Electric cars
www.modelsport.co.uk