Your mission statement is far from precise.
As for the older piston twins: Still cheaper to operate than any turboprop on missions less than 500nm ( don't get me on details here). Easier to get pilots, class rating vs. typerating on a turboprop. Don't try your luck on exotic types unless you have a very special mission that the very aircraft fits. The MU2 never saw light in europe. Even the cheyennes are getting old and it's becoming more and more difficult to get spare parts.
The KA90 is still more expensive to operate on most legs than a 421 as the early versions lack speed. The 421 has one advantage for the passengers and that is noise. You won't come close in a 414 plus its a runway hog. (depends on your mission - again)
There is always a reason for an aircraft to be cheaper than others on the market. Either its more difficult to handle (e.g.C421), or the operating costs are higher.
Whatever aircraft you operate: Make sure it flies often enough. On piston engines corrosion is a big killer and can only be avoided by flying it on a regular ( no, not once a month) base. The only way for these engines to ever make TBO is flying them frequently.
Chosing the right airframe is one part, the other one is avionics. It sure looks great to have a ton of instruments in the office but who will repair it? Spare part availability comes into play again. The more common the avionics the better.
Don't expect to buy a cheap airframe and put in a bit of maintenance work to make it fly/look good again. A thorough prebuy inspection is an absolute must. Don't save money here. Paying a maintenance engineer of your choice 5k$ for the prebuy is nothing compared to the **** you may run into without a proper prebuy.