Start by reading all manufacturers brochures and online info .
Companies make a variety of equipment in different price ranges . Most
have a low end single speed 80% furnace with cheap low seer AC. Then top
line 93-94.5 % efficient furnaces with VS DC motors for more comfort ,
humidity removal, lower electric usage and very high seer AC units. It
depends on where you live- your heating cooling needs and cash you have.
If you live where it gets -20 to 100f and it is humid look into the most
efficient equipment, Some areas will never see a payback because temps
are to moderate. You need a good instaler, a load calculation and price
comparisons on different equipment . It is smart to know what you want
first, many uneducated instalers shy away from VS DC and 2 speed
condensers. Ive had several try to talk me into simple non efficient
equipment and one refusing to sell it to me . Insulated metal ducts are
worth the cost.