I haven't been a big fan of powered loudspeakers in PA applications in the past, but for a new band that doesn't have all day to set up and tear down gear, it's a pretty handy feature.
Don't get a passive mixer though! You'll really need a good active mixer so that you'll have more control over the inputs, some basic EQ and the ability to run it from out in the house if you get someone to be the band's sound mixer. With prices as low as they are (a Yamaha 10 channel mixer for $100), you're not saving much by getting a passive mixer. What happens if you have microphones and line level inputs? Can't do it with a passive mixer alone!
I'd be happy to help if you like. Although I haven't designed a PA system in years, I still buy plenty of pro audio gear and have plenty of knowledge that comes from doing something for a long time. I could suggest a couple of mixers to begin with.
The Alto lineup looks pretty good. Start with a TS112A and a couple SM58 mikes, get another TS112A, a mixer, then add a TSsub18 and/or another pair of TS112As for more punch/coverage. That's what I'd do anyway...