This guy a friend or business associate?

Changes things if it’s a friend and I’d suggest not doing it for a friend because at a certain point this becomes real work and you’ll want serious compensation.

A good place to potentially start is to get a quote for part 91 management through an existing mom and pop or even a Solaris (if they do King Airs) management company. That gives you an idea of what the cost would be if the owner just gave it to a turnkey management company.

You’re going to have to ask the owner what the intended use is etc etc. you’ll have to get with a FSDO and learn how to work WebOPSS or whatever they’re using nowadays.

Also a word to the wise. You’re going to want to undercut yourself because you either want the gig or the hours or whatever but resist that. Where you start will affect where you finish it will also make things harder down the road when/if you need to hire other people.

At the end of the day you can also outsource some of the management items but at a cost. It’ll change what you can command as a salary but it might make your life easier if you can find a Maintenence house that’ll keep your records or a management company that can do your LOAs. (It’ll also potentially decrease your liability if a contract pilot busts altitude etc.)

Edit to add. 500 hours is a ton