I am presently in the process of buying a HVAC system, HP & AH
and Tstat. (System will replace 20+ yr Carrier 2 1/2 ton with oil back up)

What constitutes a good instalation?

How do I know that it has been installed correctly.

In my discussions one contractor offered to put gauges on the system. Should a sight glass be included?

Another offered to put flex connectors and acoustic lining to supply and return plennum.

What should the contractor supply me with to show that it is installed properly.