I would recommend a copy of IEEE 446 Emergency and Standby Power Systems for Industrial and Commercial Applications. The standard is 12 years old, but the principles are still good.
The peak power and pf are definitely important. However, another issue is starting large motor loads. I don't know if your peak reading meters are true peak, or a billing type 15 minute sliding scale. If 15 min sliding scale, then you are not seeing a true picture of the motor starting. One mfg suggests .5hp/kw.
Humm, I just made the assumption your load was lagging pf. Maybe it's not - if leading yu really need to let the gen mfg know. Most gensets don't deal with leading pf.
Don't forget to add the gen hotel loads. External cooling fans, building heat and lights, fuel pumps.
I tend not to load gensets up past 80% plus any needed capacity for motor starting.
Don't forget you will need a fuel tank. 500kW continuous is 800gallons/day. Since you are looking at a gen, which is not UPS, this tells me your outages are frequent, long, and expensive. Do a risk analysis before you buy a little (<5000G) tank
I don't tend to trust on-line calculators. The first priority of these calculators is to sell machinery. I figure they are about as truthful as a car salesman.
From what you have told us, a rough guess is a 1000kW unit, maybe $2,000,000. That's a serious enough project I would get some specialized help.
carl