Swapping out furnace 7956A856

[Skidmark]

I want to swap out a 1990 Coleman 7956A856 with a like unit. I looked up DGAA056BDTB (natural gas) Will this unit fit without modifications? It will be my first furnace swap out, but I was an auto technician in my 20s and have done home renovation for 10 years. I think i can do it, but if there is any issues I should know about up front let me hear them. Also I'm considering the precharged AC coil and lines with a new condensor to match. So Cal location ... I think our gas pressure is around 7 or 8 psi. If this is accurate will I need to worry about adjusting the furnace? I could call out an HVAC tech for this before firing it up. That's the one area where my skills are not over lapping with HVAC skills.

The existing furnace is missing the lower front panel. Is that replaceable? (can it be ordered?) I have considered cleaning the unit since it doesn't appear to in to bad a condition. No real rust issues. Coil appears OK, but Mice have nested in it for some time (The home is a vacant foreclosure). Blower motor worked but no AC. It will be a re-sale property and a new system may help market it. Is this unit beyond its expected life at 21-22 years old or is there good reason to try and salvage it? Are NEW models that much more efficient that this is a no-brainer?