Heat pump is not a good choice in your area unless it was maybe a geothermal one. A regular air heat pump won't work at -20 and you end up using the electric heater ($$$).
The guy that says he will do a load calculation, did he take any measurements? If not, he probably won't do a load calculation unless you choose him, even then you need to put it in writing. Ask the other guys if they will do a load calculation.
From what you said, you seem to have a leaky house and leaky ducts that are also undersized. Maybe try budgeting for them to fix your ducts at least. Wouldn't make sense to put in a brand new AC and have all that cold air warm up before it gets to your room. I would almost recommend putting wall furnaces and window AC system... you might get more comfort from those if fixing the ducts will be out of your price range.
You can call me Sam
It should be a crime to be a mechanical engineer in San Diego
Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7