I know the ClientExec that comes with the reseller accounts handles account creation, billing, and support tickets from your clients if desired. It should automate most of the work load on that side of things, but you will still need to setup the modx installs.
One thing you can do granted I haven’t tried it is put a copy of the zip in a special folder on your server. All new accounts get a copy of that folder which is normally used as a temp page for new clients, but I think it could be used to eliminate one of the steps for setting up sites. You would just need to extract, setup the DB, and install.
Anyways if you have any questions on the technical side let me know.
AMDbuilder
Oh, the Site5 forums are great granted modx still has the edge