If you have a lawyer friend, ask them to draft you up a clear and concise contract that covers such issues as completion time, cost overruns, warranties, etc. If a problem develops years down the road, you'll have something legal to back you up. Get a second opinion. If the price quote seems inordinately high, or you question the individual's abilities, seek out another consultant's opinion. Reward exceptional work with free referrals.
Many environmental consultants market themselves by word of mouth.
To keep them in business, tell your green friends about their performance - the good deed will eventually come back to you.