Hello . I have attached a picture of my furnace. If I am wanting to put a 4" filter on this unit, what would be the best place to put it?

Hello all, I am new to this forum as I have an issue with my Nordyne GC3BA-036C-B heat and AC system. This summer I had water issues with it, but the AC has been working fine since and I didn't have the need for heat; as the impending winter approaches, I decided to start it up and it wasn't working at all. I could see issues with the circuit board, which I replaced. I am getting a steady red light (waiting for heat request); as I request heat, the blower starts right up, however the igniter does not turn on. I see the red light blinking twice. The gas valve does not open. The double blinking light should mean (from a google search) issues with the pressure switch. I removed the grey rubber tubes from the pressure switch, and a drop of water fell from it, which is not a good sign; beside that drop of water, the hose connectors were clean. I removed it and re-installed; after re-install, I got no blower and 3 blinking lights; something might have broken with the pressure switch during re-installation, which is fine as I was going to get a new one anyway. I unplugged one of the wires to the blower (bypassing the pressure switch), and it started correctly. I will order a new switch, however my question is: why is the igniter not starting up? Do issues with the pressure switch prevent the igniter from starting up at all? The burner unit is a bit rusted out as you can see from the below picture, but when I test the igniter (multi meter set on ohm, connected to the two igniter plugs) the needle jumps up without problems. Here are some more pictures. The flame sensor looks pretty clean, so I didn't remove it to clean it The igniter also looks pretty undamaged from the water issue. The transformer shows signs of rust but the circuit board is powering up correctly, so I am not sure if this is the issue. Any input would be greatly appreciated; thank you in advance!