Once you build your site, you can add up the size of all the pics and text and get a rough guess for how much bandwidth each user/visit will consume. Obviously your site will change, but just try not to put up uber-quality pictures and stuff. Scale down the pic quality for the "web version" of any pics so you save bandwidth there.
Unless you are storing a large database, 10gb of disk space is probably overkill by a lot. If you are gonna host something that takes up that much space, then a lot of users will eat that up with the 250gb/month transfer limit.
Also, "moderate amount of hits per day" means different things for different people. Also, a simple hit isn't the same as a real visitor.
I suppose you also need to consider what type of internet connection your users will have. If a lot of local people use only modems, maybe no high-speed in your town, then you won't be hosting large movies or anything because they can't even see it... So all that goes into the site design and cost considerations.
All in all, $30/month isn't much if you are gonna make it back in advertising costs. Better to be safe than sorry. You don't want users locked out because your server and/or bandwidth is all used up. If you start doing e-commerce, then you can really worry about cost because credit card charges and all the stuff that goes along with e-commerce is gonna jack your bills up a lot more than $30/month...