Make the scope identical
Write the scope yourself in a few sentences and send the same text to every contractor. If each one quotes a different job, the prices mean nothing.
Every quote should name materials by brand and model, list what's removed and disposed of, and state who pulls the permit.
Red flags
More than a third of the total requested up front. Cash-only. A price that expires today. No physical address or license number. Unwillingness to give you the insurance carrier's phone number.
Change-order terms should be in writing before work starts — that's where budgets break.
The lowest bid
The low bid is worth a second look, not automatic rejection. Ask what's excluded. Usually you'll find the tear-off, the disposal, or the permit missing.