One thing to keep in mind, stay away from any host claiming to have "unlimited" anything, especially bandwidth and storage. There's no such thing and it's just a lure to get you to buy from them.
Back when I was looking for a new host, I found myself at Here It was a really awesome forum about different hosting companies, small and large and you can chat with other folks who can give you some good advice and their experiences with hosts.
It's also where I found MDDHosting and I personally love them. Been with them for over a year and it's been great. They are a little more expensive then your "unlimited" hosts but, I think the price is worth it. They have excellent customer service, great up time, and they're honest about the amount of bandwidth, file storage, emails, etc that you get with their packages. There's no shifty business.
For what you're looking to do in your example, you want a host package that will give you a good chunk of bandwidth, especially for the amount of members you're hoping to one day have. The coming and going on a site like that will eat up a good chunk of resources. Also you want to have access and the ability to create databases on the host server (for let's say a CMS, user information storage, etc). Most hosts allow you to do that, but some limit how many you can make so watch out for that.
300,000 members and you're deep in dedicated hosting territory. Once it becomes a problem, you will already know a lot more about this kind of thing so I wouldn't worry about it too much. Start off with shared hosting (the cheap packages on any site will do) and go from there. That said if it were me I would consider the scalability issues now rather than wait for them to bite me in the ass.
It seems like you don't understand the different types of hosting.
Most popular are Shared Hosting, Private Hosting, and Dedicated Server Hosting.
Shared hosting is best for smaller sites. Basically, multiple sites are hosted out of one server and share the resources of that server. You aren't guaranteed a certain amount of bandwidth, but you pay less. Although hosting companies typically advertise "unlimited bandwidth and storage", they'll usually cut you off when they feel that your resource use becomes "excessive".
Private hosting is much like shared hosting, but with guaranteed minimum bandwidth and storage capacity. It's best for sites in the medium size-range. A server is taken and virtually divided to create multiple virtual servers. Private hosting is more expensive than shared hosting, but cheaper than dedicated server hosting.
Dedicated server hosting is the cat's meow of web hosting. You are paying to rent an entire server and are in charge of the software on it. Best for sites in the medium to medium-large range. Be prepared to shell out some cash for this type of hosting; it ain't cheap.