The bitrate is the rate of data transfer, and you will generally find that they higher the bitrate is, the better the sound quality. You also have different audio formats that can be used - with Shoutcast, you can use MP3 or the far superior AAC+ formats. Stream hosts don't generally charge extra for customers to use the better AAC+ format because it's still using the same bitrate.
The number of slots a Shoutcast server has is the number of concurrent listeners, so if it has 50 slots then it can support 50 listeners at once (the 51st listener will get a 'Server Full' error unless they are from a reserved IP in which case some other unlucky listener will get chucked off the stream).
As CaroNet, it is a different market but it may be worth searching through your customer domains, see if any of them look like radio stations and contact them.
As far as pricing goes, it's a good idea to see what others are charging and set your prices depending on the quality of your reseller service and which part of the market you're aiming for.
