Well, this is quite the broad topic, and not an area of my personal expertise, but I'll give a general stab at it. As for how much wattage you'll need in terms of output, I'm not sure - others can weigh in, but I think I can give a general idea of the components.
If you look at completely packaged systems (like the Fender Passport), you don't need to read any further... just find the various ones available on musiciansfriend / sweetwater, and perhaps do some research around here to see what people have to say about specific models... usually people have bought em, and can attest to how well they performed in various situations.
Most non-packaged yet reasonably sized systems consist of the following chain:
1) Your mics and other instruments ->
2) A mixer of some sort ->
3) Power amp/amps ->
4) PA cab/cabs
First we'll start with the mixer, since it is what mashes everything together into a mono signal (or stereo, depending on the situation) that the actual PA portion of the system pushes out.
I'll assume the two of you are both singing, so there's two channels. Then, the keys are probably stereo, so two more (or a stereo-input channel). The drum machine? Probably stereo as well, but I've never used one. Let's assume stereo output. So, you need at least two mono chanels and two stereo.
At this point, you need to ask yourself if you want a mixer that is passive (has no power amp built in), or one that does. Im sure there are advantages to both - I personally stick with a mixer and a separate power amp, just for the ease of switching between different ones for different venues (I have a small mixer for lone shows, and a big clunky 32-track desk for recording big shows, so separate power amps mean I can swap desks as I see fit - you won't be worried about this at first though
)
So, for powered mixers, you have things like the following:
http://www.musiciansfriend.com/product/Yamaha-EMX212S-8Channel-Powered-Mixer-?sku=630192
This is a front-loaded powered mixer, that gives you the inputs necessary, and then you just need to wire the output speakers directly to it, without worry about the power amp, since its built in.
If you go for a non-powered mixer, you run the full gammit of mixers available. Most will suffice for your needs, like the following:
http://www.musiciansfriend.com/product/Yamaha-MG124-12Input-4-Bus-Mixer?sku=630049
It has the right amount of inputs for some expandability, and has stereo output, so you can run the L & R main outs to the inputs on a power amp.
The power amp then runs out to your speaker cabs. I use 2 1x15" cabs at 250 watts each, and they fill just about any room I've played.
For actual power ratings and what size of power amp you should get, I can't say. I'll leave that to the other people hopefully reading this post, and don't be afraid to search the forum, you'll find plenty of info despite this being a predominantly recording-oriented site.
Any more questions or need for clarification, feel free to ask!
