As I understand it Physical therapists help patients recover from injuries to regain their full range of motion and reduce pain. Occupational therapists provide more specific therapy to help patients perform tasks of daily living, such as dressing themselves or feeding themselves. You can specialize in one or the other, or put both under one roof. 

You could also take your show on the road and work with people in their own homes. 

Your choices should be reflected in your app. 

Provide your clients with a daily routine, specific instructions, motivational reminders that help them get up and get going, ask questions, and monitor performance. 

The job of a good app is to keep you connected to your clients on a daily basis without having to be there in person. 

20 years ago I was building websites because people were on their computers and working on laptops. Today those laptops are gathering dust because everyone is on the move, on their phones and downloading apps. 

"Expensive" is a relative term. It partly depends on the lifetime value of a customer to your business and partly on your cost of customer acquisition.

When the numbers are right, the expense becomes an investment in your business growth and the benefits outweigh the cost.