The Carvin suggestion is good... any powered mixer will work. Yamaha has some good offerings. Try and stick with a company that does higher end audio offereing as well. Soundcraft, Yamaha, Carvin, are all good examples.
Also in a round room keep in mind that you want to avoid putting any more sound energy against the walls that you need. A speaker that has a 15 degree tilt would come in handy... get it up high and point it down at your audience a bit more. An example of this woud be the
JRX112M from JBL is a good example of this, has both straight and tilt. The work as good mains when you are small and convert into monitors when you grow to something requiring more power.