As much as you should always consider your budget before making any financial decision, you should also look out for general contractors who are below average market price. Underbidding raises some obvious red flags that you must not ignore. A professional and legitimate general contractor should have a business license, liability insurance and paying their taxes all of which cost money which can only be made from the income made through the business. So, if the general contractor you are considering is giving you a low quote, chances are they are probably cutting back on the basic requirements of a professional general contractor. They probably are not paying their taxes, or they do not have insurance. In some other cases, they might be planning on charging you more as the job progresses which will cost you more than their initial quote. You should avoid general contractors like these.
When hiring a general contractor in Longwood, it is standard practice to pay some money as a down payment for the project. What you should never do, however, is to pay the complete cost of the project upfront. A legitimate contractor should have a credit and supplier accounts through which they can purchase materials to execute a project even if the client did not make full payment. If you come across a contractor asking that you make full payment for the project upfront, that is enough of a red flag to make you drop that contractor.
You could, however, create a payment schedule and release payment to the contractor in milestones when each stage of the project is completed. It is much better for you to pay as you go instead of paying the entire cost of the project at once.
A professional contractor in Longwood will provide you with a contract with details of the project and appends their signature on it. You should not hire a contractor that works on verbal agreements. A contract makes the obligation of both parties clear, and everybody knows exactly what is expected of them.