Kenne Bell SuperCharger

This ranks as one of the most asked questions on these forums, search around and you'll find plenty of info.

Summary of your questions though:
The kit will probably have everything you need

Any RWHP over 400 makes your engine a time bomb waiting to explode, if you want to go over 400RWHP safely you need to forge the internals (pistons and rods replaced that is, the crank and block are good to 600) and NO, high compression ratio is not what you want with a blower, you want low compression ratio.

You'd need COLDer spark plugs, like NGK TR6's.

The instructions have pictures with every step detailed. They are very good. The kit is very complete adn the blower is awesome. With that said, you will need colder plugs, as stated above, and I didn't like the KB supplied gauges so I went with autometer cobalts. You will have trouble getting through to KB if you have questions or in my case, want to order a part. If you have tools and a basic mechanincal ability I would say do it yourself.