$5,000
At some point, you've got to let the six-cylinder go. If you are going to plunk down $5,000 at one shot for more power, this is where we'd draw the line. Sure, if you built it up little by little and just wanted to get that something extra out of it, you could add the aluminum HESCO head on a stroker block and probably get 325-340 hp out of the stock engine architecture. Odds are very good, though, that at this power output you will be resorting to the old-boy network of emissions testing because it's likely you won't pass a sniffer test anymore.
For $5,000 in one shot, we'd go with a V-8 similar to the one listed for $3,000, add a rebuilt transmission, transfer case adapter, and the junkyard wiring, computer, and intake. Or, if you've got an older carbureted Jeep, go with an aftermarket 4bbl intake and carburetor. Seeing 350 hp and 350 lb-ft isn't out of the question.
$8,000
For this price, you should be able to get all-new stuff. It might be basic, but new is good. You don't have to worry about previous owners screwing up the wiring in the junkyard harness or any of that jazz.
If its emissions legality you're after and need to go to a newer-year engine, look toward Turn Key Engine Supply. The company provides all the accessories, wiring, computer, and sensors you might need for your chosen engine. Whether it's a 330hp 5.3L Vortec or a 390hp 6.0L Vortec, typically all you have to do is get motor-mount adapters and run three to five wires to the kit for a running, smog-legal engine package. Add on the transmission of your choice and an adapter to your transfer case, and you're ready to go.
If you don't need emissions legality, look at the GM Performance Parts catalog for whatever engine suits your tastes. From carbureted to fuel injected, there is something in there for everyone.
If you absolutely have to go with your stock inline-six, go with Hazel's choice: stock low-mileage short-block, HESCO head, Avenger supercharger running about 10 pounds of boost, custom-forged pistons at a 8:1 compression ratio, 505 performance H-beam connecting rods, higher-output injectors, bored throttle body, cold-air intake, header, high-flow cat, and after-cat setup. With an estimated 400 hp, if you've got an NV3550, you shouldn't have a problem with that power. Your axles and transfer case might start to complain, but it can be done.
$10,000
To this point, you might have noticed we've focused on GM engine swaps. The reason is simple: the small-block Chevy has the most aftermarket support and, as such, prices are lower than other manufacturer's engine families. Also, the prices are lower than the GM big-block family of engines. If you've got 10K sitting around, you are at the lower edge, performance-wise, of a big-block GM engine in your Jeep or a fairly well set-up small-block from Ford or Chrysler.
Conclusion
Face it, with this much to throw at your Jeep, there are way too many options for us to list here. We could tell you what we'd do, but where's the fun in that? We'd rather see what you come up with on your own.
Remember, pay attention to everything that touches the engine-whether it has to do with wiring, accessories, transmission, engine mounts, radiator hoses, or whatever-swapping engines is an involved task, and careful planning is needed. Do yourself a favor, plan on half of your budget for whatever engine you are looking at. Write that down on a piece of paper and then walk out to your Jeep, pop the hood, and crawl all over it looking at everything that is going to change for your new engine. Much of the time with adapters, wiring, accessories, and all the other little things that go along with it, the actual engine is only about half the price of an engine swap.