Running sound from the stage is a bad idea no matter how many are in the band.
Channels: Add up the total of how many mics, DIs, aux inputs (iPod, CD player, etc.)you need & add 50%.
Powered vs. unpowered: You're probably not going to find a powered mixer with enough input channels for your band. An unpowered mixer gives you a lot of options & system configuartion possibilities. A powered mixer limits the speakers you can add or upgrade.
Auxes: With 8 players, chances are you'll want several different monitor mixes. Pre-fader auxes are usually used for monitors & post-fader auxes for effects.
Speaking of effects: On-board (i.e. built into the mixer) can be handy, but they're often limited, poor, or both. Boards with on-board effects usually make room for them by taking out something more useful, like an aux or 2. I like to run outboard effects back into input channels rather than effects returns, to take advantage of the channels' EQ & routing options (see channel count, above), which usually isn't even a viable option with on-board effects.
I like having a lot of flexibility when it comes to putting together a system for a particular gig. I have unpowered analog mixers, a couple racks of outboard effects, EQs, etc. that are set up so that I can use 1, the other, or both, & all powered speakers that can do duty as main tops, floor wedges, side fills, etc. as the situation calls for.
Don't forget mics, cables, stands, adapters, etc. It's pretty easy to spend as much, or even more, on "misc." stuff as you do on a mixer or on speakers.
Also, don't forget to plan for a vehicle that can haul everything!