Actually LH, the general rule of thumb is that if the aircraft will do more than about 200 hours a year owning is cheaper than chartering. This is because the overheads associated with an AOC under JAR-OPS 1 are enough to make people like Daifly cry into their beer..
EESDL, if you have been using a King Air in the past then buying one is a perfectly viable option. They can be flown single pilot, and are dependable workhorses. If it's mainly just your boss and small groups that need moved around, then the light jets (CJ series, Premier 1) are actually cheaper (less expensive!) to operate in the long run, and can do just about everything a KA can except land on rough strips all the time.
As Daifly said, though, there are lots of traps for new operators so talking to someone who's done it before could save thousands of pounds and many grey hairs. Feel free to pm me if you want to talk to someone who has tarted himself for most of the motor racing operations in the country, but isn't advertising for his company!!
Good luck,
BJJ