Have you looked into a used CE-750? A CE-650 can have big maintenance costs. Unless you operate from a small runway, I found the "X" to be a reasonable value given the cabin size and speed; and since you're already Cessna'd, then it would be a good fit. If lower acquisition costs are needed, then other considerations would be an IAI-1125/G100 or a DA50...and I'm sure an HS-125 would fit in there too.