12 kt cruise wouldn't be too bad for an 8 liter engine in a 17-18,000 lb boat. If it tops out at 17 kts which is about 400 hp for an 8 liter, then that means that it is pulling about 240 hp at 12 kts. A diesel at this power loading will run for many thousands, probably 5,000 or more if maintained well. An 8 liter gasser might go 2-3,000 hours.
But if you try to cruise it at 15 kts, then don't expect much more life. The real question is where did the PO cruise her? A compression test will tell you about piston, ring and valve condition.
David