Mobile App(s) Development

I am interested in opening a discussion with a freelancer about how much work would be involved to build out a personal training software system. I have references of existing apps that I would like to look at adopting features from to build an offering more specific and user-friendly for clients and for coaches (initially just me but with the option to scale and include other trainers and fitness professionals). I would like it to also have a web-based system that is directly linked to the mobile applications so it can be accessed from desktop and mobile, at least by the coach.