The reason why the installer is so important is because most houses
have crappy installed HVAC, even new ones. A good installer will be
able to properly size, fix ductwork problems, etc. Now if your
existing system is working perfectly and you can buy one the exact same
size and install it yourself, and pay the pros do the startup and fill
freon I say go for it, but that is probably the rere freak exception.
As another example, my house probably now has an oversized furnace
bacause it had no insulation when I bought it, and I insulated it Even
if my system was working perfectly before (it wasn't) I certainly will
want to make sure that any new system is properly sized and can get
proper air flow to every room, etc.