“What would the interface look like for my customers? Would it say Dreamhost or can I customize the look? Will they login through the DreamHost login?”
1 - Looks like the Dreamhost control panel login - like this.
2 - You can’t customize it. You can assign privileges to other WebID’s via your webid panel -> billing -> privileges.
3 - WebID’s are free and they login using a WebID. FTP/Shell access is via an ftp client etc.
What plan is best for you? Compare from this list and select the best plan