Just for a point of reference, when I was playing in bands, my KEYBOARD rig alone contained a 200w per side amp running stereo through Yamaha speakers. For live PA applications, you would be best served to buy the biggest amplifier you can. You can always turn down. Eventually, with a small amp, you will run out of "up". Then the tendency is to turn up the input levels, which leads to clipping.

Read up on amplitude vs volume. The short version is that amplitude level (I guess you can call it the gain stage) is the level you send into the amp. Volume is the amount of power you push OUT of the amp. Your amplitude is finite. If you go "into the red" you start to clip (distort). Rather buy more wattage than you think you'll ever need and give yourself the headroom.

You can find mixers and amps all over the internet. Craigslist and eBay are a gold mine. A typical quick and portable rig would be an 8 space rack with a mixer (4 spaces), a power amp (2 spaces), an EQ (1 space) and a digital reverb (1 space).

I don't know your level of experience with this stuff, so I will explain that a "space" is a 1 3/4" standardized unit of measure, so a 2 space unit is 1 3/4" x 2, or 3 1/2" total. Thus an 8 space rack, holding 8 x 1 3/4" is 14" of total space. Some company makes a really nice 12 space tabletop rack for like $30. I had 2 of those before I bought my rolling server rack that has 44 spaces. That allowed me to mount a full sized mixer and a bunch of 1 space units in the rolling rack.