Greetings,
I’m going to upgrade my current furnace. It’s currently a 80000 Btu furnace with 2 ton ac. 
1470 sq ft living, 12x8 ducting. Furnace is in the center of supply and cold air returns, about 15’ each direction. 

House was built in 84 (split level) with good attic insulation and ok wall insulation. I’m going to reinstate the floor above the garage living area. 
The house has one return downstairs-600 sq ft
One return in each Ben upstairs and one in the living area, which I had moved.
I’m about to start a house remodel, I had the cold air  return moved so I could remove a wall. I’d like to go high efficiency so I could vent out the side of the house, I’d like to remove the vent going upstairs to make more room in a bathroom.

On to my questions
Should I keep the furnace 14” or get a furnace with 17.5 cabinet.

I’m going to do a 60000 high efficiency furnace and 2.5 ton AC or 2 ton ac. I’d like to go to 2.5 ton ac as long as it won’t short cycle

If I changed supply lines coming off garage ceiling to 7”, would that help with air flow. Basement gets a bid cold in winter as well (one return)

Should I add another return in the hall?

-side note, I was remodeling this past summer and got the filter incredible dirty. Line set froze as a result. Just wanted to throw this out in case I should be considering this when upsizing the a/c or just that the furnace had a very dirty filter.

Any advice would be very appreciated

Advice on new furnace.