I'd recomend motor first, then lift and wheelbase. Just my .02, but the lift and wheelbase might seem a little wierd if it doesn't go anywhere.
The 20R, 22R and 22RE are all very reliable. 20R, 22R are carburated, RE is EFI, depending on your preference. All are fairly affordable.
I'd recomend calling around to Pull n Save type places to see if they have a motor. That should save you some coin. However, you may still end up with a rebuild depending on the condition of the motor. Again, time consuming, but not really expensive.
Perhaps you will get better responses if you detail your experience and level of aptitude for this type of project. My previous suggestions involve some working knowledge of engines to be feasible for you.
Baby Beast- Mini Truck in progress