Paul in Hawaii most of the small property management companies that handle vacation rentals charge 20% to 35%, the larger companies (i.e. Outrigger, Aston) that often handle grounds and building maintenance, will charge 50% to 55%, of the rents and have a front desk, maintenance crew, and staff.
What reservation or channel management system to use? I know many of the smaller property managers are simply using vrbo, airbnb, etc to get property exposure, handle bookings, collecting of money from guests, etc., and then they may supplement that with Excel for billing purposes. Now, I'm not recommending that's what you do, but that would be a bare bones approach, although maybe not the best for your clients if you are handling multiple properties with multiple owners.
It's been a while since I looked into different platforms such as Kigo, 365Villas, bookerville, myvr.com, etc., it'll be interesting to see what platforms others recommend for managing multiple units.
Good luck.