Welcome to owning a YJ, if you have an open mind to learning automotive mechanics a YJ is a great place to start. I've learned untold amounts about automobiles in general just from owning this jeep
Theres a right way and wrong way to troubleshoot problems, and I'll tell you the most basic thing to know when diagnosing a problem is the 4 things required for a gas 4 cycle engine to run properly:
3. Ignition (Spark)
Air is usually the easiest to rule out, is anything obstructing air getting into the intake runners?
Fuel can also be easy to rule out, typically hooking up a fuel pressure gauge to the shrader valve on the fuel rail, and listening to each injector with a mechanics stethescope or long handle screwdriver for ticking sounds, will both give a good indicator if you are getting enough fuel.
Spark or ignition can be one of the harder ones to figure out, but with a few tools and the right knowledge of how your motor fires up, you can figure out the source of the problem without "throwing parts at it" so to speak, and losing money in the process. I typically say test the most common parts to fail first, in this case, spark plugs, and wires. If you just bought the jeep its not going to kill you to do a tune up, IE replacing the plugs, wires, distributor cap and rotor. This is a good idea, however if this does not fix your problem you will obviously have to dig a bit deeper!
Good compression is rather easy to figure out as well, typically you use a compression tester which will give you a read out in PSI. You can also do a leak down test. This can let you know if internal problems exist.
What other symptoms are you having? Does the jeep crank but not start?
As for the leaf springs, stock YJ springs are generally flat, as long as they haven't inverted they should be just fine. Most people like stock springs as they give a smoother ride, and are typically retained in spring over axle lifts when good flex is a concern.