I operate a 1980 F90 Part 91 and can give you a few data points. We do not fly the airplane enough for me to give you a very good total operating cost, in my opinion. It would be pretty skewed to the high side due to our low usage.
Insurance--we are just a shade under $10K a year for single-pilot, Part 91 ops. Piloting is not my primary role in the company, and I'm told by our broker that hurts us some.
Performance--I cruise in the FL200-240 range and flight plan 75gph and 260KTAS. She'll do this all day. You can get her higher, but with the stubby wing, it takes forever to get there and you are kinda hanging on the props.
Phase Inspections--This one is tough for me to pin down exactly because we almost always have some other inspection, avionics work, or some other minor thing going on at the same time, but I'd guess we average in the 8-10K range for two phase inspections per year.
What breaks--Not much. Other than recurring inspections, the T-tail inspection hole SB, and routine wear items, we have had very few issues in our almost 9 years of ownership. The 6-year gear inspections can get pricey, because, as I understand it, the F90 only shares main gear structure with the 100 series. The 200/300 gear will not interchange so you are dealing with a very small parts inventory for a very small fleet. Same with the props. We had to replace a few original blades this year because they were at their minimum thickness and it was not a fun conversation with the boss. You can get blades from Raisbeck, but they are more expensive and there is no gross weight increase STC for the F so there isn't much advantage over factory Hartzells. Look for one with recent prop overhauls and preferably new blades.
All told, it is a great airplane for our mission of hauling 3-5 people over 500-700 mile legs. It has the best electrical system of the small KAs, the 200 fuel system which is pretty much foolproof, and good speed for the cost. The boss loves the cabin, and I absolutely love to fly it. I have flown J3s to C-5s and the F is one of my favorites. It is also the sexiest King Air on the ramp in my biased opinion.
Good luck in your search. If you find a good one, you will not be disappointed. I just hope you get to fly it more than I do ours.