The condensate line all the way to the draft inducer needs to be kept clean. I would run the intake pipe to the outside.
The draft inducer has some ******* where they measure the capacity make sure the ******* are not clogged w/ stuff.
I do have a Heil Furnace but from 2004
My friend did something like what you trying to do except he used a wood burning water heater.
Put some wood on it, let get get to temp and pump begins to work.
I have one but it needs constant TLC like every 3 months and you need to replace the energizing wires when the break.
You better off going a 4 inch filter and MERV 8 or higher would be nice!
Here in the mid west cold country, I normally buy the biggest battery that I can fit on the car. I hope by doing these I would get a higher CC and more reserve. Most people just buy the battery at Walmart but I get mine at Menard's. Batteries are like water heater only few manufacturers make...