Depending on budget levels, you cant go wrong with a Losi xcelorin combo.
both sensored and sensorless are available for a really good price at
www.dms-racing.com
use sensored if you plan on doing competitive racing or sensorless if your are only bashing and dont need precise power control.
other options are to keep a lookout on places like ebay for some good prices on seperate controllers and motors etc.
I picked up a new team orion 70amp controller and xcelorin 10.5 motor for my stepsons xx4 and only paid £72 for the pair.
others have already answered on Lipo's, but keep in mind when buying you will also need a good balance charger and leads to keep the lipo's working at peak perfomance, thus adding to the outlay.