I would also like to find out the same. I have seen a couple of options on the market (although they all seem to be sold out or discontinued) and they seem to be around £100, which to me seems excessive considering you can buy remote control cars, etc, for less than £20 which should have all the stuff required in them (although charging the batteries through the track would add an issue / expense). It's not much money when compared to DCC but I just don't want to feel I'm massively over paying for something.
In terms of which locos to convert. Any of the old Lima bogied diesels should be OK as they have a mountain of room inside their bodyshells and most can be made to run pretty smoothly - they also emit quite a nice motor growl as well.
I've been trying to convince my friend who is an electrical engineer to 'knock something up'. He says it would be quite easy, but he can't see any money in it. I will try again though.