I own a 1990-FordRanger-but I plan on buying a newer (used) model soon and would like to know which years have known mechanical issues. I've heard some years have head gasket and transmission issues.

I'm interested in getting a 2000 Ranger with supercab or newer.

 

Well I got a 2000 Ranger XLT Super-cab with a 3.0 and 255,000 miles.
The only major work I've done, was replace the intake manifold gasket cause it was leaking water and replace the heater core, it was leaking also....lol but I think it went bad around 225,000
My truck is still running to this day. I drive it just about everyday, also.
Hope this helps
What do you want to do with your current Ranger?

 

Some years have inherent issues but most depend on how the vehicles were taken care of.

If well maintained you should be ok with any year.

Look for a newer year than the 2000, a lot of the problems with the 1998 - 2003 truck were taken care of.
One of the newer V-6s is a good way to go for more power.

Try to see any maintenance report the owner might have and if a dealer get as long a warranty that you can.

Luck