Lots of good info above. I have been toying with renting myself (minus myself as DJ/sound tech), but it just scares me too much. And I already have a bunch of low-cost mikes that people can bounce; I just don't want them too. There are so many scenarios that could go wrong...
I would definitely go with credit card; that's the way it's always been done when I've had to rent anything in the past. I'd also have paperwork that they sign accepting liability for any issues (thus the additional credit card run). If you are not using excess gear that you already have, and are considering getting something specific for these lower end rentals, I'd think about something extremely simple, maybe not even powered speakers (two sets of adjustments, trying to match volumes, etc., etc.). Maybe a simple, 80-100 watt Sunn or Fender or Kustom 4-channel head with very minimal controls (they can actually get quite loud with the right size speakers-- I've done graduatuions in large gyms with some of these smaller amps), some speakers, tripods, mike, mike stand, mike cable, speaker cables... plus the "special" mini-to-RCA adaptor. I wouldn't give them any additional help re: music service, live teching, etc.-- it would seem too much like competing with yourself and your colleagues based on low-ball price. I know you want to catch those gigs that slip through the cracks, but maybe there's a downside to offering a quick-ticket to an iPod wedding on the website?? I don't know...
GJ