I owned an SMB in the health & fitness space and spent several years running a SaaS/PaaS startup in the business productivity space. There are lots of WP developers out there, I'd start by looking at elance. Have you done mockups so the developer can accurately quote the work? It's worth a little design expense up front to make sure your vision is communicated clearly.

I'd also make very certain that an off the shelf solution won't work. Zenplanner and Mindbodyonline are two cloud-based CRM/studio management solutions tailored for fitness businesses, I assume you've ruled those out. There are many generic SaaS CRM solutions out there as well so I'd really make sure that the "ideal" experience is really that much better than "good enough". I used MBOnline and it was not perfect but it certainly didn't hold back my business.

I'd also recommend finding someone else in your space who has what you want for customer experience, and just ask what they use. Assuming they are not a competitor they will probably be forthcoming with the info.

Happy to jump on a call if you'd like to discuss further.