I used to buy and sell these things...more as a hobby for pocket money than for a living. So here's a brain dump.....
Much of your choice will come down to budget.
While planning you have to consider what will be used while there is a power outage. Critical circuits only or what you'd typically use with utility power. Start-up load is very important. A well pump may only consume 4kw of power when running but may require 8kw to start. There goes the 5000kw (Surge, not continuous) rated Home Depot generator.
IMO the best choice is to go with NG if it's available. Diesel units can tap into a home heating oil tank for fuel...there a bit louder and smelly but live a very long time. Gasoline is messy, can go bad and may not be available and you have to transport in small quantities. Propane is expensive and requires tank refills.
Output from a NG unit will be less that the same running on propane or gasoline. However it's a bit quieter and you'll likely never have a shortage of the fuel.
You want the lowest RPM machine you can afford. For home back-up I would not buy above 1800RM. Lower RPM ranges are from 1100-1800. Other units are considered by many to be "Lawn Mower" engines. They're louder and comparitively short lived. These units are typically sold at Costco, Home Depot etc and run at 3600rpm. Do they work? Yes. Will they suit your needs? Maybe. Will it last as long as an Onan, Winco or higher end Generac? Not close. Hmmm, how is it that the discount store 15KW units are 20-30HP and the "commercial" level ones are 50-70HP?????
Transfer switches are what isolate power sources. Manual units look like big Frankenstein switches. Automatic switches are great but as you go up in Amperage the amount of the check you're writing gets much larger. Again a choice you make. Is it critical that power be continued if you aren't present? Do you mind throwing a switch, starting the generator and vice versa when the utility is restored? Traditionally 200-300 amp manual tranfer switches were in the thousands of $$$. My Square D lists for about $1800 (a good price). However, I've seen equally rated units at Harbor Freight for $200-$400.
Below is the 4 cylinder/65 HP Onan I have on a trailer.....sold to a gentleman in CA. Waiting to get picked up. I'll replace it with another similar Pre-owned unit (cost $1000-$3000). These things can handle a good sized house with many appliance running concurrently. Take the advice from others.....get an electrician to help plan and install.....further, an electrician that has experience in this field or else you'll end up being a guinee pig.
Edit: Remember if looking at used generators for a home you want 1 phase / 60 Hz.
