If you don't have a budget for better speakers, then sound treatment of the room is probably out of the question as well, so you have to deal with what you've got. If it is possible to put a sound deadening drape behind the band, I'd strongly suggest it. It will make a world of difference for you!
Your biggest problem in a room like that is keeping stage volume quiet enough so that your PA speakers can actually do something. If your drummer goes all animal on the drums, there's no way you'll be able to maintain a mix at 90db.
For micing the drums, you probably could get by with just one mic in the kick. Even that could be optional. However, having mics on the drums means you have the ability to tweak their sound to some degree which can make mush turn into something a little more pleasant. In this size room, amplification isn't your goal, as much as clarity adjustments, especially if you're targeting 90db.