Even if these contractors aren't scam artists, they may lack licensing and insurance. If a worker gets hurt on your property it could wind up costing a lot more than you bargained for.
Instead, get contractor recommendations from friends, neighbors or relatives. Check references and get documentation of insurance coverage.
And don't put more than 10% down for the job. Mete out the payments gradually as work is done and withhold the final 25% until you're satisfied with the completed project.