(summary at bottom)
I've been away from dealing with webhosts for a while (aside from a friend letting me use 100MB/1GB for $.50 a month to play with

). Coming home from college, however, my mother has a project for me. She is a realtor, and she is currently paying $30/month for her 'webpage'. There are a lot of people that take advantage of real estate agents and offer them a 'simple' solution for a $300 setup fee and outrageous monthy fees. [my mother is currently using one of the 'cheaper' services]. Anyway, I can create basically the same service for her at a much cheaper cost. Simple databasing and user-friendly forms and I've got a marketable service.
What I came here to ask, is what should I look for in a host? I can't see these agents using 5GB a month, maybe the busiest sites getting 10GB. They also don't need very much space. They run 3-6 listings at a time with a few pictures and some text for each, nothing major. Nobody would need more than 50mb, as long as I keep the servers tidy.
Basically, I don't know how well this is going to take off. I'm going back to school but it's an easy service to maintain from a distance. I know I should have at least 3 clients (+1 community site they want to share). But, the way word spreads through these offices, if these people are satisfied with the price/service they will drive more business.
What I'm looking for is a host that can provide me with multiple domain hosting and an easily upgradeable plan. How should I go about this. I'm thinking that down the road I might want to look into a dedicated/managed server, assuming this picks up between the area offices, but I don't think I need to make the service capable of handling that yet. What is my best plan of action.
Summary: I'll be running a service that will originally demand <20GB transfer and <200mb disk space, but may grow rapidly. What should I do about hosting?