Hi Robert
First of all I'm the highest rated European photographer on this site and have access to plenty of suitable models. That does mean that you will get professional work. Please see my portfolio.
What you seek is 15 images in a minimum three different locations; I have an award winning park, a set of skateboard ramps and an "interesting" beach front and river mouth near me as well as the South Downs National Park on my doorstep. I'm wary of including identifiable mobile devices without the manufacturers' permissions.
Now to the copyright bit.
This site has a standard copyright release section in its T&Cs and you will be granted release of digital rights on settlement of fees through the site's escrow (milestone) system.
Under UK law (where I am, and where the photos would be taken) and under Australian law (where this site is based) your document would actually have no legal standing. You can't impose conditions on jobs administered through this site. Having said that I have a good enough reputation both here and on other sites to be respected for delivering what my clients need. Original copyright, at this fee level, would remain with me. In any case under UK and Australian law, no model could legally sign your document ... only the photographer. I know that's probably not what you would want to hear, but that, unfortunately for you, is the way it is.
Please discuss on chat
Barney Douglas
Hangleton Lane Studio
Worthing, West Sussex, UK