Sell: Multi-Domains and Domain Mapping

Let me get to the point,
I want to sell websites to my clients with their own custom domain. I want to get them on monthly payments. Right now in order to do that I believe I have to have:
-A Sub-Domain installation of Multi-site?
That should allow me to properly use multi-domain in conjunction with Domain mapping?

The reason I went with a sub-directory installation of multi-site was because I wanted to avoid getting more and more SSL certificates, but if I am unable to use multi-domains then its oK.

SO, in summary:
My main website is: http://www.dosedaily.com
My clients future site: http://www.healthclinic.com
I should be able to do that under my main dose daily.com site? And my clients will need to get their own SSL certificate if they are accepting payments?

I will also want to use Pro-sites, ad-sharing, white-labeling, and mobile phone app support.

I am asking these questions because I want to start off at the right base with less issues in the future, thanks for your time sire or madam.