Busking amp from prefab parts

Hey I know there's a few threads about this already, but they seem to be dead, and i just had a few quick questions.

Normally I'd have no issues in knocking a system together from more elemental parts, but i'm really looking for a quick, simple, and reaaaaly cheap way to do this. I want to run a guitar, bass, and vocals to the one amp to make for better transportation.

My new band won a bit of money in the local battle of the bands, and I wanted to use my cut to build a portable busking amplifier so we can continue to gig on a regular basis for money. So I read this how-to on building a cheap portable amp and really liked the idea of using a car amp. Since this is a budget street operation, I'm not looking for any high quality parts, so I was thinking a really cheap car amp from dx . Now of course I need a preamp, and was thinking something along the lines of this baby here. My main question is this, do i need to run a new preamp for each channel, or can I mix before the preamp, and cut costs? I have a feeling there is going to be issues of impedance if I'm mixing a mic and guitar signals.

Though really I'm out on a limb here and would take any advice I can get. Thanks for at least reading this guys!

tl;dr: Can I mix mic and guitar signals before the preamp?