That will be a shared server, not your own, so you won't get all 30GB and all processor time.
Zero, few places show the full server specs on the rental page, they will only show memory/slots. They will usually have a page to say "these are our systems" as well, but not straight there, because it will not affect the performance. I have been hosting servers for years now and still do, I am yet to get a CPU bottleneck from using countless hosts across multiple games, because they factor for it.
I would look at two options:
Firstly, renting the server, you will want 3GB+ RAM allocation for 30 slot, which is not 10 bucks a months cheap if you are with a reputable host. That also depends on mods, with 8 of us playing Tekkit and FTB we were hitting our 1GB cap, so bare in mind more mods = more performance drain
Secondly, renting a dedi box. You get the box all to yourself but have to set it up all by yourself. If it works out similarly priced, or even slightly more, go for that. Performance will be better, you will be able to expand more, will be able to host the game and a TS/Vent/Mumble server, website, FTP/redirect and any other games you want, as well as rent out extra space to others to cover some of the cost, because you will not fill it on your own.
On a 2GB, C2D box we had a Minecraft server 2 CoD4, TS, website and were renting out 2 other CoD4 servers to others and another TS and we still had enough left for another few CoD servers easily.