The frame is rusted.

I think not , for the money you could find one in better condition .

Typically, 4WD trucks sit high and front tires splash up at the area around the front seats. Check to see how much rust is there. If it's just superficial, maybe a cleaning and application of extra protection is all that's necessary. If rust has done slight damage, maybe weld in some reinforcing. If extensive, you won't want to rebuild the frame of a truck that's only worth three grand.