hi,
ok since you do not need to record, then a normal mixer will do, no need a USB one. the interface that i mentioned earlier is for recording purposes as its USB/Firewire.
mixers come in different sizes, from 8 channel to 12 to 16 and much more.
based on the amount of instrument u want to 'combine' together, you'll choose the number of channels the mixer have. usually a 8 channel mixer has about 4 XLR impute, which means you can put 4 mikes and the rest will be line-in, which you can put your guitars and keyboards.
depending on your budget, you have a range of brands to choose from, like mackie, yamaha, etc.
for me, i'm more familliar with yamaha as i'm a yamaha user, a 8 channel mixer cost about 200+ to 300.
if you go to yamaha at PS, u can see mixers that are silver in colour and mixers that are blue in colour. those in silver are USB ones, means it has interface incorporated. for you setup, you'll probably be looking at the blue ones(analog mixers), which are more suitable for live and cheaper also. the MG series.
you can get them at SIM LIM which has better prices, i recomend 'World Wide Sound' shop at level 1.
website to see the mixers
http://www.yamaha.com/yamahavgn/CDA...%253D560489%2526CNTYP%253DPRODUCT,00.html#one