As has been suggested, estimates aren't the only thing. Get references, check online, check with your state to see if they're licensed, insured and bonded.
If an un-insured painter falls off a ladder on the first day and breaks their back, your homeowners' insurance will be liable. This is what happened to the sister of the accountant at my lumberyard.
In my area, my County Health Department has a list of reputable (licensed, insured, good work) contractors that do this sort of thing. When I had my system installed, i called around for estimates and was surprised that one of the largest companies that does mostly large scale/commercial groundwork was a very competitive bid, and they got it done very quickly.