Are you asking if 20GB of disk space and 300GB+ of bandwidth is worth paying $4.95/month for? Or are you asking whether or not you'll actually be able to push that much BW with HostGator? Or, maybe something totally different?
i dont think hostgator will let you use that much bw without throwing some rule in your face after you're around half way there. Bandwidth isn't cheap and $4.95/month for 300gb is cheap and no host would benefit from that deal.
You're better off getting a VPS where you will have non-oversold bandwidth thus, less fear of being shut down.
7c.) Backup Limit
Any shared account using more than 20 gigs of disk space will be removed from our off site weekly backup with the exception of Databases continuing to be backed up. All data will continue to be mirrored to a secondary drive which helps protect against data loss in the event of a drive failure.
So, space wise, it would see they do let users have more than 20GB of data in their accounts, or they would have little need for that clause, but when it comes to backups, you're on your own.
I suspect that the bulk of your usage will be determined by static files (wallpapers being downloaded), so you've got a fair chance of pushing high amounts of data transfer with one of the budget hosts out there. I would contact the hosting provider though, explain my usage, and ask for their advice.
VPS would be the best option for you. If you care about pricing
search for "cheap vps web hosting" in Google there are a lot of sites that offering good price on vps hosting not to mention you will not have to worry if someone using more resources that cause your site down and so on...
well you should first contact that host about this and then host with them.
Hostgator have a limit of 250,000 iNodes which means you cannot store more then 250,000 files total.
However i suggestion you to get a VPS server