Generac, Kholer and Briggs & Stratton and even Champion are all good brands. Champion the max size is around 12kW.
The price looked good at first, but it does not include the ATS. But the prices are WAY up! I'd add about $1K for the Transfer switch. You also need to figure at least $2K for the electrical hookup and $1-2K for the plumbing hookup, depending on the distance.
Getting your electrical service replaced at the same time will definitely get the attention of an electrician. You might get a discount for the work. Get several estimates. Some of the generator install companies have the plumbing and electrical on staff or they have "captive" contractors.
I also recommend you have soft starts installed on your Air Conditioners. Soft starts are NOT PTCs or hardstart kits. These lower the inrush current to about 1.5 times running current. I run a 16 kW generator and these allow my generator to start and run both of my Air Conditioners (4 ton and 3.5 ton). I really like the Hyper Engineering units.
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On size, the 20-22 kW air cooled units are the most cost effective. They are also quieter than the smaller air cooled units. The units that start at 25kW and up are usually water cooler and are derated GM vortex car engines.
All of these generators are designed to run for 100 hours between oil changes. They prefer high quality synthetic oil. Keep oil, oil & air filters and spark plugs on hand for changes. You really should change the oil and filters every 100 hours of operation or at least once per year. This matches with your car oil/filter changes. 3000 miles at 30 miles per hours = 100 hours! You can get away weekly changes during longer emergencies, but I recommend it. Some people just don't know and they might run for weeks and kill the generator.
BTW, portable generators without oil filter need their oil changed every 50 hours!