Thank you for the great answer! That was exactly the kind of advice I was looking for. I have to admit I always looked at gears and chains as trivial mechanical issues, but now a new world opens up in front of my eyes.. Also a world where, like I find in your experience, I think I couldn't afford a car if I had to buy its individual pieces 
..
Anyway, I made my mind to give Tamiya's kits a try even if, considering I plan to reduce a stepper motor, I guess some careful adaptation will be unavoidable.