Hi Ian.

How tight is a tight budget? One option is to get a couple of the little gear motors and run them off the servo boards in standard servos (£5 each on sussex-model-centre if I'm not mistaken), though the motors are still a bit more pricey, you get a much better performance from them.
If you're going down the servo route, my faves were always SD200's. Hard to speed mod, and the gears tend to strip fairly easy, but they did me well in so many robots, and my best ants all used them. Also recommend the park flyers, heavier, but easy to speed mod and great for a weapon servo.

I've got a micro reciever you can have if you want it. Its on 40mhz and it's no real use to me, would be good to give it to someone who's gunna use it.