Re: Interface help
Ok, to be real honest, $100.00 is going to be tough. Yes, you can go Behringer interface below that, but you're going to need at least one mic. LOCAL second hand is going to be your best bet here (no shipping). If your going to be doing mainly vocal recording, get a 58 used. Cheap, works, don't need to worry as much about background noise / reflections etc and will be better than a cheap condenser. Works fine on guitar too.
software-wise. If you want a full blown DAW Reaper is the way to go for cheap, $60 for a fully functional DAW. Keep an eye out for sales or coupon codes (although i don't believe they do that much). There are TONS of free plugins out there that it wont be an issue at all.
EDIT: What computer are you running? Asking as one of your concerns of the interface was to 'handle' all the instruments etc. If you mean by ins/outs then that is all interface, but having 48+ tracks, all with plugins is going to depend on your computer specs. Just something to think about as well.