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Anything I should look out for when I go down the scrap yard?
You'd be better off making your own servo system.
I tried using a window motor on my 150lb robot but it just didn't have the power and the current draw was too high.
That's just way too funny. You call it a 30kg robot but you say it may have two car batteries? Those alone would be the weight of your robot! As it is on my own robot I'm using a battery half the size of a car battery, and it weighs 20lbs, or almost 10kg.
Also I wanted to make a note that your robot isn't going to be cheap with an electric drive train. I'd find what kind of motor you wanted to use first, because if it draws over 100 amps you're looking at over a hundred dollars just to control it--probably more money than the motor itself will cost. That's one of the reasons why I like gas engines--but ironically I still plan to use an electric motor on my robot for backing up! At least it won't be of the power you're probably thinking of making it plenty cheaper.
Also, ro-bot-x states a 5 amp controller will be enough to control it, but I would advise a 10 amp system. As it is my 10 amp system isn't quite what I want as it has no room for the motor to slow in rough situations. This is why I think it would be best that you had at least 10 amps, and with a 24v system hopefully it will work great.Also, you made mention of a servo you thought would be strong enough to power the steering. Well, I hate to break it to you but under no circumstance will that servo be in the least capable of driving the steering on a robot that large. Here is something for you to contemplate; on regular cement my steering motor uses maybe 7 amps to turn the wheels. At 12v, that's 12*7 or about 84 watts to power the steering. Now to power that small servo you would probably need to regulate your power supply to about 5v. 84/5 equals 16.8 amps to get the same results out of that motor as I get out of mine. To put it into perspective, most 5v regulators are only capable of outputting 1.5 amps, so to get enough power out of those regulators you would need 12 of them, needless to say that will fry your motor. Hope this helped you contemplate the complexity of what you wish to do.
Anyway, hope this helped and good luck.