Since most directories starting up do not have a lot of traffic, you usually would have nothing to worry about.
You have a point.. so 'only' when you directory starts to slow down.. you should consider moving to a better hosting.. I see.. scale the operations only when you require scaling I think I have my answers now.
A very good point there, I'd like to add that you can do your homework with any number of hosting providers and prepare the ground ready for if and when you do start getting traffic, you'll be surprised at how receptive some hosting companies can be.
If it's a set and forget kind of directory, you should use the most affordable host you can handle. Dreamhost works ok.