Can this be added to an existing site with a word press blog already installed? Example: www.abcd.com/blog?
If the site already has a load of posts and you wanted me to keep them but add all the goodies I could do it but I would need to charge a little extra because I could not do this within my current system - PM me with the site details and I will take a look for you. I probably won't scare you.
Also if not how do you determine what the domain name should be? From the niche site you build or do the buyer have to determine the domain name?
Usually if you are getting a new domain name then I'd try to buy one that was using the core keyword within the domain name so if the site was to be about 'panic attacks' then panicattacksinfo.com would be a useful sort of domain name to go for. Try, where you can to get a .com domain.
And last if one has hosting with unlimited what would be the next step after purchasing if none of the above is feasible.
The sites are always built on your own hosting so we'd simply build the site on your server. So you'd simply choose a niche, order the service, buy a domain name to match and point the domain to your existing server. If any of that is unfamiliar territory to you I have provided a whole load of videos including several that cover exactly these steps in a very simple to follow fashion.
I look forward to working with you.
Thanks for asking questions!