Issues that have been brought forward are quite legitimate
ie. warranty, quality of install etc.
To alleviate some of your concerns why don't you e-mail
the manufacturer and ask them if you are covered?
Their is one distributor (DESCO) for Goodman who claims that they are an authorized distributor for Goodman, York etc. and your warranty will be honored under certain conditions.
e-mail the manufacturer to see if thier claims are true.
You then can look for a tech who can do the install.
If you are confident and handy you can install a thermostat run the proper wire, have a sheet metal guy fabricate connecting
duct work, get drainage line installed, have proper electrical connections ready, equipment in the proper place, maybe run a lineset.
But before all this spend a few hours reading hte install manual
Is it worth it?
You will have to decide