There are several variations in the LS engine family.
First you have the Gen 3 and Gen 4 engines. Gen 3 were generally between 1996 and 2007. All LS coded engines had aluminum blocks, some of the non performance engines also had aluminum blocks. Most of the trucks/vans/SUV's had iron blocks.
Gen 3 was the LS1/LS6 and the 4.8-6.0 truck engines.
Gen 4 was between 2005 to now. In the Gen 4 engines you had the LS2/LS3/LS7/LSA/LS9, these all had aluminum blocks and were the performance versions of the Gen 4 series. On the truck side, there were 4.8-6.2L versions. Same story with the iron and aluminum blocks. Some were, some weren't.
Many of the parts between the Gen 3 and Gen 4 engines are interchangeable. Also, regardless of engine size, the blocks were the same physical size. If you can fit a 4.8, you can also fit a 7.0L.
As for power, any of the LS engines can make 400 - 450 whp with only an intake, cam and heads. The larger displacement engines obviously will make more power and torque. I've seen some 4.8L engines put down 450 whp and rev out to 7,500 RPM. I've also seen people get a complete 4.8L pull out with 200k miles for less than $350 and throw a turbo setup on it and make 1,000+ whp and run it for several seasons before the engine let go.
Essentially, the things to keep in mind are the reluctor wheel which is either a 24x or 58x, oil pan (most swaps recommend the 98-02 fbody pan), drive by wire vs throttle cable and aluminum block vs iron block and finally, what transmission are you going to run.
Expect to pay between $300 - $600 for a decent 100,000+ mile 4.8 or 5.3 pull out complete with wiring harness. The larger engines and aluminum blocks likely will cost more.
If your car came with a 4.6, you could just buy adapter plates for really cheap and then it's just a drop in job as far as getting the engine mounted in the car, not sure whats required with a V6, I would assume getting a K member and dropping it in. Wiring and all that fun stuff is it's own thing. You could even just go with a carb setup for cheap if you wanted to.
It's really a straight forward swap, as far as engine swaps go.
Here is a decent thread showing an LS swap into a 94 Mustang V6 - 94 Mustang 5.3 LS Swap - Ford Mustang Forums : Corral.net Mustang Forum