First, welcome to the Forum!
If all you’re trying to do is set up your HT system, there’s no compelling reason to use REW. At its most basic element, “setting up” the system merely involves adjusting the relative levels of the speakers. You can do that by ear using the test tones your receiver has. Or, if you desire a bit more precision in level setting (most of us do), you can use an SPL meter instead of going by ear.
The purpose of REW is for fine-tuning beyond basic level setting, to get the best in-room frequency response from your speakers (among other things, if you want to go even further than that). Obviously peaks and dips in response are not pleasing to the ear, so the more of them we can eliminate, the better. Most people are satisfied at this level by smoothing out their subwoofer response. REW lets you see exactly where the peaks and depressions in response are; the BFD smoothes out those problems if placement isn’t able to take care of them. The only thing you need to accomplish this is a RadioShack or Galaxy SPL meter, a USB sound card, and a few cables. The RS meter is the cheapest option and works just fine for subwoofer response; if you can’t buy one locally I’m sure you can find one on eBay.
If you want to move up to taking accurate full-range frequency response readings, that’ll cost a bit more money, because you’ll need a calibrated mic and maybe a pre-amp (in the event that your M-Audio won’t work with REW). We have a recommended vendor who sells the Behringer ECM8000 for a reasonable price. Even if you don’t resort to outboard equalization, full-range readings can help you set any tone controls your receiver has, should they be needed to smooth out the response of your surround speakers.
Hope this helps.