There are lots better choices.
If you absolutely must have a piston airplane, you should look into a Colemill Panther conversion. Just call Colemill (I don't work for them) and they'll send you an impressive and comprehensive video presentation that will convince you it's the way to go, although more costly than buying a stock PA-31 off the market. However, if you do go off the market/stock you will either fly around a dog with some issues you don't even want to deal with, or spend on maintenance in the first year what you would have spent on the Panther anyway. Trust me on this.
However, you really should do a job on your principal and try to convince him/her/them to bite the bullet and buy a King Air 90. At least 60-90 knots more than the piston PA-31 line and pressurization, plus simplicity and reliability, and great resale value...plus a very comfortable cabin. There's a reason the King Air line has been in production since 1964 and is the only business turboprop still on the market (and I don't work for Beech/Raytheon either).
If purchase price is the controlling issue with your principal, see if you can steer him/her/them into a Caravan, or better, a PC-12. Single engine IS a safety issue, however intelligent operating procedures, chiefly some wiggle room on weather minimums and opting for the less congested departure/destination airfields, will go a long way to ameliorating this concern.
If you do go with a turbine aircraft, do your department a favor and stay away from Cheyennes.
Best of luck and feel free to PM me with further questions.
--best, beach.