The model tractor you choose for your homemade rc tractor is probably going to be the biggest factor in determining what functions you can add. You will most likely want to pick a tractor which already has steering, SIKU and britains tractors seem to have good steering racks built in which makes things much easier. I have only used SIKU tractors so far myself and I found that some LEGO bevell gears suit the steering rack perfectly so you might find that useful. You can always cut the gear of the steering wheel and use that on your servo to steer the tractor, you'll probably be cutting it off to get it out of your way anyway.
Another feature of the SIKU tractor I liked is that the engine can be remove i.e. the tractor and engine is not one solid dicast block which I think might be the case with britains tractors so beware of that when you pick your tractor. You might also wish to pick a tractor with a full bonnet so you can remove the engine completely and put whatever you want in the engine bay.
Given that the biggest issue with build a homemade RC tractor is space you'll probably want to pick a pretty big tractor, I like to use 1:32 scale tractors and any of the models you see on here will be homemade 1:32 scale tractors. That doesn't mean you need to do that, there are many people who build tractors to 1:10, 1:14, 1:16 scales pretty much any scale you want.
The advanced rc tractor can be done in a few different way, to add funtionallity you can buy an rc controller and rc reciever with more channels 6 or eight and use those to control lights and control accessories. Another way to do it is to use a microcontroller in the the tractors, this gives you much more control but is much more complicated. This is the method I like to use mostly because I like working with micro controllers.