J.M.
I have a 10 month daughter and I use cloth diapers during the day, pampers overnight. Right now, I have 8 medium size diapers (4 brand fuzzibunz and 4 brand bumgenius). I have fuzzibunz snap diapers, bumgenius snap diapers, and bumgenius velco diapers. The velcro definitely gets the best fit, but I know people say they wear out faster than the snaps will (although I've used them for about half a year without any problems). I haven't tried any other brands, but I think fuzzibunz and bumgenius are both very good.
According to the size charts, medium is supposed to go to 30 lbs, so that should be the only size you need. I think people buy the one size diapers so that they can use them from birth (medium would be way too big for a newborn), so I think you'd be okay with medium or one size (I've never used one size). The ones I use I think are called pocket diapers, when you buy them the inserts are included. I'd occasionally add an extra insert, but usually I just make sure to change my daughter's diaper often.
I think the cost is about $16-$20 per diaper. I bought all mine online from Target.
Here is a link with a video on how the fuzzibunz works: http://www.fuzzibunz.com/details.php