if you're considering one of those 500 gigs for $7 plans, check the terms closely to see if there is a limit on the total number of files. stored email might be a large number of small files.

if you managed to get a couple hundred gigs of email stored there, would you have a way of moving that data to a different webhost? what about keeping backups?

Be careful with email on the cheap shared hosting. The one i use decided one the day to put in this drastic spam filter that resulted in many emails being lost totally and the rest delayed by days. I ended up switching to google apps... If you go with one, read their terms of service carefully, they may only guarantee service for web hosting and not email.