Re: how many watts for vocals?
There seems to be a lot of opinions on this. Some based on fact and some not, some on experience and some not.
I did have a sound reinforcement company for a while so I will only speak from experience. I also did some install work on rehearsal studios.
The first thing to ask is how much money you have?
The second is can you figure out how to use the equipment correctly?
The difficult part of a sound system for rehearsal is the same problem as a monitor system. It's feedback.........since the mic is pointing at the speaker. That limits the amount of gain you can use, thus limiting the overall output.
That's why it's important to base your system on power and not gain.
Whatever you decide on I would suggest looking into a feedback suppressor and a compressor. I would recommend the DBX 166 compressor or for less money the 266, they are the best for the buck. As for the suppressor/eliminator DBX, Sabine and even the Behringer are OK. Again it's how much you want to spend.
By using those 2 pieces of equipment you can get the most gain and control of your system making it more efficient. The compressor will also protect your speakers and fatten up the overall sound.
I would also recommend a simple inexpensive reverb unless it's already in the board.
Also some type of EQ is absolutely essential. The feedback suppressor and compressor will allow you to EQ the room more as opposed to the Mic/vocals.
The more bands the better, but I would rather have a good stereo 15 band than a cheap 31 band. Rane, DBX, Furman and a few others. Unless it's already in the board. You can also use a parametric to really close in on the problem frequencies but that's overkill for your needs.
Stay away from the inexpensive stuff, they just do not work well and can cause other sonic problems. It's the weakest link thing......great system, one crappy component ruining the whole system.
I like some of the Mackie stuff and Yorkville. You can either have a powered mixer or powered speakers. I would rather use the powered speakers.
And whatever you decide use good quality cables, again it's the weakest link and makes a big difference.
If you want more help contact me and I would be glad to work out the details so you do not waste money and get the biggest bang for your buck. There is no problem buying certain things used if you know what you are doing. Most good power amps are indestructible and those older one's from Crest and Crown can be had cheaply.
I dunno that I'd cringe hearing Paul Butterfield wailing through a blackface Twin Reverb though...I'd jump at the chance if he was still around. They were great! 
