Assuming your well pump is a 5 hp pump, running amperage should be around 28 amps which means starting amperage could be as high as 60-80 amps momentarily. Generators don't handle motor starting well, so size your generator accordingly- at least 50 amps on the output which would make it about 10-12 KW. Nothing wrong with a manual transfer kit on a panel since you will hardly ever use this machine. Get an electrician who knows what he is doing to install the panel in the pump house and the manual transfer kit on the panel and you'll be as safe as you need to be. If needed, heat in the pump house can be as simple as a keyless porcelain base fixture with a heat lamp in it, or a small baseboard 120V heater. You definitely don't want to take a chance on freezing your well head if you live in an area where this could happen.
A few things to keep in mind- if you won't be using this generator often make sure to exercise it . Once a month for at least 20 minutes would be best, but at least several times a year they need to start up, run until warm, put a load on them for a while by flipping the transfer switch, and then let them idle down and cool down. A generator that is treated like this will last a very long time and be ready to be used at any time in an emergency. Let them sit without attention and there is a good chance you won't have a generator when you need one.
Nothing wrong with your fuel source plan as long as you can be sure to keep enough to be able to handle your needs as long as needed- a day, a week, a month... who knows? Try to get a generator with a good muffler and pipe it out of the pumphouse through a fireproof thimble. No matter how quiet it seems your generator runs, without a decent muffler you will quickly tire of listening to the drone of the engine.
Be sure to get a few extras with the generator- a battery charger that keeps the battery ready for use at all times and plugged into a receptacle in the pumphouse, a block heater if you live in a cold climate, and an automatic shutoff on the generator in case of low oil, over heat, or other dangerous/damaging situation like smoke/fire or CO in the pump house. An alarm to the house the case of any of these emergencies would be a good idea and fairly simple to set up.
The rest I'm not sure I can follow this thread to see what has already been done or considered so I'll leave it here for the moment.
Last edited by Sheister; 11/22/22.
