I just rented a car from him for 2 weeks. I've been renting from him for about 6 years because his cars are in better shape than other car rental's and his rates are pretty reasonable. Two years ago I had a small problem near Villa Vasquez - I had it fixed at a mechanic and he credited me back the cost of the repair. I'm sure if you break down near Sosua he would send another car, but if you are farther away I don't know how practical this would be. For insurance, I usually give him an imprint of my credit card which insures the rental through Visa. If everything is OK after I return the car he rips up the imprint and I pay the balance in cash.
If however, you are worried about insurance and emergency service, you may be better off with Avis or one of the larger chains as they may have better facilities for this. Obvoiusly if they have a network of rental locations across the country they could be more flexible with sending help.