The S-15 Jimmy was introduced in 1983 with some unbelievably underpowered engines. For instance, one engine option was an Isuzu 2.2L diesel making a measly 58 horsepower. Remember, this is in an SUV.
By the end of its run, engine options were improved, with some making close to 200 horsepower. The Vortec 4300 V6 CMFI sold in 1994 was the top engine offered on these SUVs.
Owners of these vehicles have been nagged by a series of little problems that may or may not require replacement, depending on how important it is for the owner to have a vehicle that is problem free. The door locks were a constant problem, mostly due to poor design, including a piano-wire type mechanism and weak springs. The tailgate handle is also known to break rather easily. In many cases, the lock will cease completely, requiring a tool like a flathead screwdriver to be jammed into it to open it. That can be rather embarassing if you're trying to impress someone with your SUV.
The 2.8L engine offered on first generation models was also a big problem. It leaks oil so abundantly you'd swear they design it that way. Swapping that engine for a 3.4L or a 4.3L from later models is a good solution, and the parts to make the swap are very common. Look to pair the 4.3L engine with a 4L60 transmission. Many owners using that drivetrain combination claim to have 200,000 miles on their vehicles with no serious problems. Not bad for a GM product.
You can find these Jimmys all over the used car market. You can also find dozens of listings on eBay. Most parts can be swapped with the aforementioned Chevy S-10 Blazers, so there are also a ton of parts out there. Don't overspend, and don't think twice about buying new parts off eBay instead of paying a huge premium at your local parts store.