I am not sure how big in terms of the number of resources you will need or how large your viewership would be so it would be a little hard to advise.
However, as for hosting two domains on one account, it is possible. Some hosting company does have this has a standard feature while others offered it as part of a reseller's account.
Note : the Reseller's account does not necessary mean that you need to resell web services. In fact, some people sign on to them because they offer the best value for money in terms of the resources provided.
If you are looking at setting up databases, yes, it is possible. On the linux environment much of this is provided via mySQL database system. I am not sure how you want to set up the database, ie how you want to collect their information and how you want to utilize them. But I am sure there are ready-made scripts out there which should meet your needs.
Take a look at http://www.scriptsearch.com/Perl/Scripts_and_Programs/
I am not sure if there are web host catered specifically to create celebrity fan sites but I believe the major components would be basic descriptions, photos and perhaps sound or video clips. On the interactive end, maybe a mailing list or a forum. Some side features may be the hosting of a e-card system or something.
Most of the scripts are available. If you do not know how to set them up, I am sure the authors would be more than willing to do so (for a fee).
So in short, look for a web host which provides :-
100 to 150mb web space (for your pictures/texts and so on)
CGI Support (for your web programs such as forums and stuffs)
MySQL Support (so you can install database-centered online cgis)
Probably a bandwidth of about 10 to 20gb to begin with
Supports 2 or more domains
Of course, most importantly, the ability to grow if needed.
As a side note, I trust that you have the permission from the celebrity or his/her/their/its managers permission to use some of their resources to use on your site.
Yes, the budget of $50/mth would be possible. Even if you offer 1/2 of it, you should still be able to get offers for it.
Do a computation on how big your site would be in terms of resources in kilobytes or megabytes and add an allowance of 50 to 100% to that so if you need to grow, you can.
To determine bandwidth, you have to guage what the target audience would be and how frequently they would visit multiply by the size of your resources. I know this is something very difficult to guage but as a benchmark I would usually use something like 20 x the size of my resources.
So if I have 100mb of files, I would need : 20,000mb or 20gb bandwidth to be on a very safe side. On a conservative end, maybe 5 to 10gb would do as a start. Then guage month by month what the growth trend is.
Once you do up those calculation, gather all your requirements and hop over to the 'web hosting requests' forum and mentioned them and you probably would get deals which would surprise your wildest dreams.
Hope that helps.