Just picked up a 1999 E-150 with 242k miles for only $600. My girlfriend and I plan on converting it to a race hauler for motocross. The only problem is a little rust on the rocker panels, runs a little rough at idle, engine shakes when accelerating to 40-45 mph and the brakes are horrible.

I'd like to tackle the engine shaking issue first, I noticed that it idles a little rough and when I accelerate it's smooth until I reach third gear then it starts shaking. Tried shifting to 1-2-D and same thing happens. Could it just need a tune up or what. Where should I look first?