Nearly all smaller generators (30 kW or smaller) have "reconnection switches" to allow you to switch a set from single to three phase. Usually the switch is under a fastened cover to keep morons from turning them while the generator is running and from turning them at all if they are not supposed to.
Some sets are rated to run at full load all the time (power factor, or PF, 1.0) many are rated at 80% output (PF 0.
Add up all the loads that start automatically (well pump, furnace, refrigeration etc.) and add about 30-50% to that to cover starting loads and misc. uses and you'll be in the ball park for the best size for long term use.
You can get by with a smaller set if you are willing and able to manage the loads individually.
Lastly, NYS law and the electric codes require a double throw transfer switc to lawfully and safely hook your service to the generator. the only other legal way to use a generator is to connect the loads to the generator directly.
The diesel sets currently being disposed of are relatively heavy duty but are being released, in part, due to a comparatively poor history of reliability. By that I mean, if you really, really need to have a back up generator and there is great risk to life, limb or property, you might want a new commercial set.
Lance