You don't need a soundguy although it's nice, since it's very tough to set levels onstage. A low budget method to get around not having a snake is to get a couple of two-way radios for the soundcheck, and put your soundguy in the audience with one of 'em. Keep an earpiece from the radio in the bass player's ear, and she can adjust levels on the fly during the show.
300W should be enough for a couple hundred people, and more than plenty for monitors. Teach the [expletive deleted] drummer to play at a reasonable level, mic the snare and kick, and two overheads. Think about your ears--there is no need for excessively loud stage levels. Let the audience go deaf from the PA, save your own hearing.
Mic the guitar amps, run the keys direct, and of course mic the vocals. With 300W I'd definitely avoid running bass through PA. You can crank the bass amp, it doesn't cause any problem with stage levels like guitar amps do.
Set the guitar amps at proper levels to hear onstage, and monitor just the keys and vocals. If you have two monitor mixes, maybe give the key player some guitar in her mon.
Finally--and I can't stress this enough--get the [expletive deleted] keyboard player to set her volume at max for the soundcheck. Otherwise she'll increase her level during the show if she wants more keys in her monitor, unwittingly increasing keys in the PA too (this is because key players are stupid). This is a headache for the soundguy, but a disaster for a band running their own sound. This is why I give the keys their own monitor, and I crank the keys in the key monitor to fool them into thinking they are louder than they are . . . I [expletive deleted] hate keyboards . . .
Guitar and bass don't seem to have this problem, because they always play at 11. This is because they are also stupid.
Pretty much everybody is an idiot once you are onstage . . . don't forget that. Normally intelligent people become extremely dimwitted while onstage, so make your setup as simple as possible, practice the setup like you'd rehearse your set. Know it cold down from mics and cable and monitor placements to typical settings on your mixer, and you won't screw up the sound too bad.