I've both rented a car (Easyway is good and will provide all insurance needed except for glass and tire coverage which you can add for $5 per day). Cost with insurance for a small car should be $35-40 per day.
I have also used USA Transfers 3 times for roundtrip from airport to my hotel or condo, they are very good and also do both group and private tours (parent company is Entertainment Plus and they have been in business for 14+ years). There are others, Canada Transfers is getting good reviews lately as well.
PAWeber, there is also a free road to Valladolid, depends where the OP is starting from. The toll road is just south of the CUN airport. The free road starts at the first stoplight at the entrance to Tulum Pueblo, hand a right, go through 3 small villages (watch for topes or speedbumps), come to a traffic circle and take the turnoff for Valladolid, should take about 2 hours. I am not sure if the second class bus line you mention runs in that part of Mexico, though the second class line is the Mayab, which is owned by ADO. Mayab buses do not have onboard restrooms unlike the ADO buses. Have a wonderful trip.
ETA - I have been staying in Akumal almost every year since 1999 so will be glad to try and answer any questions you may have. Just got back in May this year.