I have an early 90's Trek 2200 with Shimano 105 components.

I want to get different wheels for the bike. I'm looking at Mavic Open as I hear and have read some great reviews about them. Will these fit on my bike and what do I do with the rear cassette? It's currently a 7-speed rear cassette, can I change that to a 8 or a 9? About how much total $ would this run me? Thanks!