I would say check out alot of host sites. Read their Terms of Service(TOS) to see if they allow hosting of adult material. You can start by looking at Host Gator as mentioned by Lincxu but I would keep an open mind and look at many hosts before you make your decision. Every host should have a TOS so look for the one that fits your needs the best.
Hostgator allow adult content BUT (yes there's a but)
1) make sure you are within the legal requirements
* must be 18 years (or older) this is 21 in some us states
* no adult content that involve(or display) minors(illegal)
*watch out with rape images/videos (rape itself is illegal and considerd female unfriendly)
*no pics with or videos with animals....
but i'm afraid if you get 500 members after a week allot of hosts will shut you down
Stay away from hosts that offer "UNLIMITED" Disk Space/Bandwidth. This is pure ******** and their strategy to attract more people. They basically crowd their servers with a lot of accounts which makes the servers slow and down frequently.
In the webhosting industry, you get what you pay for. So avoid getting into the traps of "Cheap Hosting Companies."
You can ask the following companies if they'll be able to host you on a dedicated ip.
If you had that many members after only a week and plan on growing, i think you should be looking for a VPS server plan. Budget will not be alot higher then your projected $100 a year. But keep in mind you will get what you pay for. If you are going to spend the time getting things up and running and by the sounds of it what you offer is popular, then pay alittle bit more and keep the site running :-) As for the adult content issue, a simple look at any hosts TOS will alsmost always answer this but at the same time you can always ask them.