Hi,
My recommendation would be to buy a carefully and properly matched setup, front and rear bars, springs and shocks. If you don't, you will be fiddling with combinations of these parts trying to tune the setup yourself. That can be both exasperating and more expensive, in the long run.
Brand is of little importance and the vendor should have sorted out what parts work well together. The more experienced the vendor is with chassis setup, the better.
Also, talk with them in detail about what you want, how you plan to drive the car and how other suspension and handling considerations will be set up. For example, will the car be lowered or not? What wheels & tires? Etc. Heck, even front to rear weight distribution can be important, if you are really looking to tune handling for peak performance.
If you want or need to do it piece-meal, i.e. install a little at a time, that's possible. Just be sure to plan in advance the entire front and rear "kit" so that everything will match well in the end. And be certain to install the front sway bar first, then add the rear later. Never, ever install a rear sway bar alone on the IRS TRs, without a front bar, or you will find yourself with some pretty odd handling characteristics (possibly downright dangerous!).
Most rear sway bar setups are mounted under the frame and end up a bit exposed, reducing ground clearance (as Shannon mentioned). A few have been worked out to fit above the frame and might be a better choice, but will most likely be harder to install. (For one, I think Revington TR offers this type.)
The bars themselves are just solid (usually) steel bars. Heck, you can even make up a custom one pretty easily if you can find the blanks to work with and have an acetylene torch. The key differences are the diameter of the bars front and rear (thicker = stiffer), the methods of mounting and the end link design. A full race setup is usually stiffer and has little forgiveness, will make for a harsher and noisier ride: using metal or hard urethane/nylon mountings and bushings, and possibly rod end joints (aka "rose" joints, in the UK) on the end links. A street setup might have rubber mountings and bushings, which allow some play before the bar starts to limit body roll. In between there are also compromise street/race setups with a little bit harder urethane bushings and mountings, or even a combination of rubber and urethane.
As already mentioned, there are also adjustable bars and end links, which can be quite useful to fine tune the handling or make it easy to change for different purposes (such as a street car that is occasionally autocrossed). Mounting hardware needs to be high strength and good quality to handle the stresses involved.
Polyrethane tends to squeek when dry, so poly bushings might best be greasable if at all possible. These might just have some grease groves formed inside where a special grease (provided) is smeared during installation. More rarely but occasionally they have actual zerk arrangements to allow a bit of grease to be injected once in a while.
Rod ends (rose joints), if used, need to be high strength and some are teflon lined for smooth operation. There are also dust seals used on some, or available as an add-on. There are some installation tricks with rod ends, to provide fail-safe. But, I doubt most kits you will find will be using rod ends.
Have fun!
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