Yes, 12, 14, 16, 18 and 20G Professional series tractors were all built on the exact same frame, transmission and chassis. There are a few minor differences beyond the engines.
18G and 20G tractors came with a number of upgrades/differences/variations. Here is a list:
18G/20G have larger tires front and rear.
ALL 18G/20G tractors came with hydraulic lifts, manual and hydraulic models offered in the 12G thru 16G models.
Wider rear rims were introduced on 18G and 20G sometime in the early 90's
18G/20G have longer steering arms and larger, more heavy duty tie rod ends.
Kohler twin powered 16G tractors did not have oil filters, 18G and 20G models did.
BUT, Kohler powered 16G models are actually 18 HP Kohler M18S engines, just like the 18G, less the oil filter.
Hour meters were standard on the 18G and 20G all along, later, after about 1995 they seem to have become standard on all tractors.
All of these tractors started out in the 80's with hand controls and right side brakes. All were later offered with left hand brakes, foot controls, optional dual wheel brakes and optional conversion back to hand controls.
Wider front rims were standard on all models starting in the early 90's.
Hope this helps. A review of the the parts book I linked to earlier, will show you the parts that are different and the parts that are the same. Example, there is a page for 18G/20G steering, and a separate page for 14G/16G steering.
Even though my tractor was built as a 16G, it has been fully upgraded to the 18G parts and specs except for the wider 18G rear rims and and tire sizes. Two things I don't feel offer any benefit anyway.
Personally, I would not mow with garden lug tires, and they are not good in snow from what I hear from others. Turf tires with chains, or something like All Trails are much better for snow.
This setup has plowed well over a foot of snow many times.