Appointment Scheduling with Credit Management

I’m planning the development of a WP-based website to market and sell my online coaching services. I am looking for a WP plugin to allow me to do the following:

a) sell (via paypal) bundles/packages of credits to my clients – (ie 12 credits = 12 hours of coaching) and then allow the client to schedule specific sessions via an online calendar showing available/unavailable times. Each time the client schedules a one hour coaching session one credit should be subtracted form their account. If they have insufficient credits they should be prompted to buy more.

or b) allow the client to subscribe with a monthly fee (again via paypal) which will add a set amount of credits each month to the clients account – again the client would follow the same process to book coaching sessions on a calendar presentation of available/unavailable times, with one credit being subtracted form their account each time a session is booked.

Both approaches should allow the client to reschedule bookings up to a cut off/cancellation period – say 24 hours prior to the booked session.

Each time a session is booked confirmation emails should be generated, and reminder emails should be sent at one or more predefined intervals prior to the booked session.

Additionally I will need an admin override to add/subtract credits manually if the client chooses to pay offline and/or cancels booked sessions outside of the cancellation period.

So far I haven’t found any plugin or alternative tool that can handle both the appointment scheduling process and the credit purchase/management process. Any thoughts on where I can find an robust tool to achieve the above would be much appreciated.