Lots of your answers are on this board or
www.snowperformance.net/forums that can be found by reading and searching. I'm a newb to this stuff and I've already got a system up and running and making ~350hp on a 2.0L Suby without a special tune.
I would plumb in the water after your SC to save wear on the compressor. I think the Snow stage 1 kit would meet your needs and budget. As far as I know you don't need to change nozzle size when changing from water to meth mix.
-Steve.