finally, somethin I can help with
you'r biggest enemy in the cost department is going to be the amperage you're going to be pulling. Most over the counter esc's (electronic speed controls) aren't going to be able to handle it without dying. This pretty much leaves you with 2 options....home build a mechanical sc, or buy a nice esc....one is obviously going to be alot cheaper than the other....if you decide to build your own you would just need the transmitter, reciever, 2 channel reciever that matches the transmitter, a couple of servos, and four leaf switches rated to the stall amperage the motor is going to have to pull, a couple of fuses would be a good idea too....as well as something to putt it all on. Another huge disadvantage would be trying to make it walk correctly....if it were me, I would just build it so it drove on specially made "wheels" so it would shuffle, then try and animate the spider so it appeared to be walking.....it'll be alot easier to build, as well as probably faster, and more reliable. As for the motor mounting and such, I would be more than happy to help you design something, but I would have to see a picture of the assembly, and possibly a couple of measurements .
I have tinkered with "robots" before, and still have a small library of literature on the subject, I would really like to see somebody make something like this, so if you need any help with anything, just shoot. I'll be glad to help any way I can.