to over simplify the issue, you have to figure out your needs and desires first.
I have a 3500 back up genny for the entire house - 2600 sq ft. I have a free standing wood stove for heat and no need for 220 power in an emergency and did not want to invest in an automatic one and have a propane tank installed out here in the sticks just for that. I do not heat with propne.
There you go. I do not know what you need, so you have the next challenge.
And on the sump pump issue, i have a water powered back up, and the supply comes from a city water tower since i live along a road to a state park.