Ok Guys here you go. This diagram is "scavanged" so it looks a little un-refined from my battery mig thread here.
You can see more diagrams so you can get the idea.
As far as soleniod a constant duty 12v starter solenoid will work just fine. (29volts gets halved and looses 1.4 volts, so halved is 13.8 volts DC or so)
You also need a rectifier (I recommend around 5 amp+) and a capacitor. Just buy the biggest one you can find at radio shack and you will be fine. Probably about 5-6 dollars at radio shack
See attached diagram (that was slightly re-worked the capicator has nothing to do with the motor speed control)