Get multiple quotes and be careful to compare "apples to apples". Ask to
see the product and for full details of the process to be used, insulating to
be done, and quality of the windows or doors themselves.
If you are uncomfortable with the thought of spending a lot of money, all at once with a company - just have them do 1 window as a test first. This will demonstrate how they operate and give you some peace of mind, before you have them proceed with doing a large job.