1. references - not ones from the installer - ones from friends or relations.
2. Installers length of time in business.
3. Deposit required.
4 Lead time.
5. Uv value
6. Where manufactured eg ROI or NI or Europe
7. Does installation include replastering reveals
8. Self cleaning
list is endless.
Guarantee important but not too important. You will be told ten years, but if you have a problem with a hinge /handle etc you will be told that there not covered.
I bought my own in Europe.
Guarantee was useless, that was a risk I was prepared to carry. So far its not been a problem.