It still runs ! Take the advise of the marinas technician Run it. Run it to another facility that will give you an honest diagnosis explaining his findings. " SHOT " is not a good explanation! The issue is finding a service center to work on the Mercuiser Cummins. Not because they are bad engines, but because most technicians don't want to get involved in the parts chase. Do your home work find all the parts availability and the find a good technician to install them if you are not a DIY boat owner.
I was involved with the replacement of the turbo on my D3 220 Volvo. It was labor intensive. The Turbo is on the starboard side of the engine. The After cooler, oil cooler lines, engine ventilation oil lines, Turbo oil return lines all have to be removed to remove the Turbo. The cost of the new Garrett Turbo was $2600.00 and change, While I had the after cooler out I took it apart and cleaned it and installed new o-rings. The parts Turbo, Gaskets, seals, hoses, from Volvo were just over $3000.00. There was 13 hours billed to the job 120.00 hourly rate.( Warranty) I did 75% of the work because the Volvo Technician was a big guy and was struggling to work in the tight quarters of the compartment. Also because I have a hard time having anyone work on my engine or boat!! While the 5K may sound unreasonable you may find that to be an average estimate if you have a marina or service center do the work.