TMG I would take a long hard look at the AC50 PA39 has a handle on.
Failing that a really nice low time C414 ex the US - I could put you in contact with trusted people in the states who could find and prebuy inspect such a proposition. They don't work for free of course but they would save you a potential fortune in NOT buying the wrong aircraft.
Turbine wise a good low time C441 ex US would be awesome - quite simply the most excellent twin I have ever flown and as easy to fly as any 400 series Cessna twin. 290kts/FL300+ and you can either take 3 bums 2000nm or 8-10 (double club seating plus crew seats) 600nm.
Or a PT6 powered Conquest 1.
Its a great time to be buying aeroplanes in the US and if the boss hankers after a Citation one of these days perhaps you should be thinking about building time, and corporate experience, that will lead towards that in a logical fashion - in fact a really good C441 is so good the Citation could go on the back burner for many years - there is a reason Cessna stopped making 441s - they wanted to sell Citations and wouldn't have sold many with the C441 still in production.
If you want to do charter pressurisation and some speed are huge selling points...as well as radar/boots/hot props etc in the southern winter.
I think you'd very soon outgrow the Partbanana and you'd be looking to change up again - its essentially a twin C206. Whats a new Partenavia worth?
Hmmm C441 - US$850k
1983 CESSNA CONQUEST II Turboprop Aircraft For Sale At Controller.com