1- this is a default config if you want these styles to be per usergroups, but can be possible via a addon if you want your users to select their own style.
2- Hostgator.com have their own official forum using vBulletin, it's a good guess if it work well there!
3- when you buy a package for hosting, you have host but also a certain bandwidth... the more you pay, the more bandwidth you have.. a startup is possible with any package.
4- there are some free styles here, you see the tab on top right of this page... but you can search google: "vbulletin style" .... for some paid ones too