2). There is no should be and 5 is preferred. Insist on a proper load run. Most homes are oversized system with too small ductwork.
3) some modifications to duct may be necessary, make sure unit is sized properly first
A poorly sized system on undersized ductwork will kill a unit quickly...
Rheem is as good as anything else out there if it is put in properly. If your ductwork isn't sized properly with too much static it will kill a vs blower, once out of warranty they are expensive to replace. Just an fyi.