I am also involved in praise & worship at my church which is about the same size that you mentioned. I monitor this site a lot for tips on gears.

There are many praises in this website for the Bose that these guys were talking about. It is an amazing technology that is hard to believe. It has equal volume whether you are 5 feet away or 20 feet away.
I believe it is already powered speakers so you would not need a power mixer. Just a regular unpowered mixer would do. Like this one below
http://www.guitarcenter.com/Behringer-EURORACK-UB1222FX-PRO-Mixer-102583614-i1153472.gc

You need to figure out how many channels (pastor, keybd, drums, singers, etc) and then for sure you need some effects to enhance the vocals. The mixer above has it all.

Guitar Center has all these things and offers 30 days money back guarantee (last time I checked) which is great since you can try them out first without too much risk.

Fran is a longtime resident and seasoned pro here. He also recommended some Podium speakers that received lots of praises in this board. Ask him about it for recommendations but I do not think GC carries it. I think he also owned Bose as well.