Originally Posted by stealthdevil
How much webspace/bandwidth do you require?
I'm not too sure they know how much they need (as per their initial post), but if the op can give us a general price range, we can suggest what disk space/bandwidth would be reasonable to have at that price range.
As pointed out, these questions would be great to send to the pre-sales team of any host that you come across that you might be interested. Generally hosts make it as easy as possible to upgrade (and usually without an upgrade fee -- just the difference between the two plans, but some hosts do charge a fee, so its best to ask the potential hosts about their exact policy).
Generally you would have to register domains for your clients as well, your host will not do this for you (in most cases).