There's nothing sacred about the seal under the AC. It's a closed cell foam ring that goes around the opening and compresses when you tighten the 4 inside bolts. It is re-usable if it isn't crushed too flat, although I have seen a couple that someone put dicor on both sides of the gasket before setting the AC. They were a mess to clean up.
There may be components in the middle but all that I have worked on stay on the AC when you lift it up.
When you change manufacturers you will have to buy an inside plenum for the ducting to match the AC. They pretty much universal.
My only suggestion would be to buy a simple one that uses a standard thermostat. None of the fancy microprocessor garbage, as they are more opportunity for failure.