100 Watts should be large enough for crowds of up to 50, but you need something with more oomph for bigger audiences. If you have more than one audio output it is sometimes possible to use a domestic system and put the top frequencies through this while also using a guitar amp for bass. The best way to avoid problems is to have a trial run the day beforehand. If you want to develop more complicated set-ups it is worth investing a couple of assorted adapters and carry spare BNC and Phone cables. There are kits available which enable you to convert every conceivable type of audio and video connector into the desired plug and these are worth the money.
Most video projectors are totally useless in a day-lit space.
Video projectors cannot deal with other light sources, so be careful when using spots and other lights in gigs.