If you want a one stop solution have a look at the Yamaha Stagepas (?sp) systems. The bigger one would fill your needs, I think. The disadvantage here is lack of "expandibility" as it were.

If you want to go "component" I've used Tapco, Mackie and JBL powered speakers. Even the Tapco Thump is fine for most listeners. The joy of powered cabs is that you only need to buy mic cables for everything. You will need a pair of sturdy stands.

Use a pair of powered cabs with a small but powerful desk with built in fx. Beware modern desks as they skimp on mic inputs. Make sure the desk you buy has enough, they are the one to use. Avoid running long line level cables to the desk. Get a traditional table top mixing desk rather than a rack style job. They are tough but usually cramped and hard to use in a dark corner. Get a desk lamp with a nice tight beam pattern too.

If your room allows it you can avoid the need for floor monitors with a bit of careful speaker positioning. Get one speaker slightly behind and above the performers and they will hear themselves perfectly. I've used this arrangement a lot due to lack of gear or setup time and even had rock bands tell me the vocal sound was great on stage (as it should be, they're hearing part of the FOH mix). Buy monitors later as funds allow, more powered cabinets will do nicely. Some prima donnas will complain about the lack of wedges but, hey they're only doing a couple of numbers at an open mic. If they want super gear they can get a booking at the Albert Hall

As Trev says, Behringer DIs work just as well as more expensive ones. In fact, they're the only Behringer product I'd recommend.

You seem to be covered for mics but I'll just say that three or four vocal mics will go a long way. Unless you have super bat ears it's still hard to beat a vanilla SM58 for most things and they are cheaper than ever in real terms.