Ummm, if it is a 4.0 then you want to make sure it doesn't have the infamous Cylinder 4 problems. Plus the intake manifold vac leaks.
Here, check this out:
2000 Ford Ranger complaints, problems & defects information | CarComplaints.com
All pre-2001 Rangers are know for electrical problems. With the 2000 it is mostly the dome light staying on in the extended cab versions and the door ajar lights. No biggee.
Personally, with mine, I have had brake problems (which are also common). The rear drums retain water like a b*tch. Then if you are braking hard and creating to much heat the metal parts break and warp. The pads and drums wear prematurely due to the water damage. Plus my rear wheel cylinders are going bad from this also. The best way to fix it is just swap in an expo rear axle or convert to disc brakes.
All minor issues. Nothing that isn't the norm on here anyway. All 98-11 Rangers are essentially the same beast.