Being a 91 it is a 7.3 IDI engine. They are reliable and most parts are cheaper than the powerstrokes. How ever they have far less power than powerstrokes. You can put a turbo on those engines very is easy just as long as you have the pump turned up. The last one I did that two was a 93 dually and it got 17 mpg pulling an enclosed trailer. Just change your oil at least every 5k along with the fuel filter. They will start below 40 deg as long as your glow plug system is working properly. For every 1 hour multiply that by 25 and that will tell the rough mileage. Even idling (like most ambulances do) equals 25 miles.
Not that I know of. The 4bt in Gruman delivery trucks had turbo 400s behind them. So they will bolt up to non LS sbc transmissions. It is a matter of making mounts, minor wiring, etc.