Considerations for installing a high efficiency furnace in my attic?
Yes you can put a high efficiency furnace in an attic hose if allowed by your local code. I use self regulating heat tape if there is any chance of the condensate line freezing.

Have you considered a mini split system? I have installed high efficiency furnaces in attics in the past but you would still need a separate compressor unit outside for AC. 

The newer mini splits are extremely efficient and zone control can’t be beat as each unit or zone can be independently controlled. 

With larger homes I recommend multiple outside units each outside unit can serve from 1 up to 8 inside units depending on the size and brand.  Mini split systems don’t require duct work and this is where installing a new forced air system gets expensive, in your case with multiple heat sources and ac you should at least consider a split as it won’t take up space in the attic or in the 2nd floor if you choose that location , no duct work is a big plus when changing heating styles like you are planning.