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.
Understand the process. Will windows be "retro-fitted" using the existing
frames or will it be a "frame-out" installation. ("Frame-out" is recommended
in most cases to ensure a good seal and the opportunity to properly
insulate, for the greatest energy savings.)
Compare delivery timelines, length of install, and if there are any potential
extra costs and get them in writing.
Compare all warranties and service guarantees and get them in writing.
Get a written, detailed quote listing all necessary materials and labour
costs. Beware of the "blank contract" or the "hand shake".
Ask for references.
Be sure you hire professional installers, who specialize in windows and
doors. Don't leave it to a "handy man" or another trade. After all - you
wouldn't let your mechanic cut your hair would you?
Beware of high pressure salespeople telling you what you want to hear, not what you need to hear. This can be a large investment. You need to do it only once and properly. Everything that is done should be what is best for you- not anyone else. If it sounds like a gimmick- it probably is.
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.