Caribe Tours website has all the information you need.
Colonial Zone hotels vary from seedy to luxury (Hotel Frances, Hostal Nicolas de Ovando). In the middle range you have places like Doña Elvira, Hotel Europa and several others. Search the forums and all will be revealed!
Taxis inside the city are RD$120 unless you get a tourist taxi outside the hotel or in certain spots in the Colonial Zone.
Taxis from the airport are about RD$800 or US$25.
Make sure you agree on a fare before setting off, and have the exact change.