Get additional estimates if you feel that you are not completely comfortable with a proposed project.
Make your selection based on overall value not just cost. Spending a little less to get an end product that you are not pleased with is no bargain.
Make sure the contractor you choose is full licensed and insured for the type of work proposed.
Make sure you understand the warranty coverage being offered both by the contractor and by the equipment manufacturer. The contractor should offer labor warranty protection for at least one year.
If coordination is required with the contractor and other resources such as the fuel supplier make sure you have a clear understanding of the responsibilities for the scheduling and payments of these services.
Make sure you have a firm understanding of the job time table and projected completion date.