Hi,
For my sins, I've got several P55's of various kinds - two marine more or less up together and another in a lot of bits, one industrial complete with radiator etc that goes and another industrial engine bare.
I'd have thought $200 (say £130) was between about right if it goes OK and rather a lot if it is an unknown quantity. If it's a basket case, forget it, if it's missing any important bits like magneto, carb etc, leave it alone, it'll cost you as much again to find them.
Spares are not a problem, but are VERY expensive new as there is only one company with the franchise.
They are not an uncommon engine, lots of people I know have got them, but very few exhibit them. Thirsty, heavy and rather boring.
Just my opinion, you understand ;o))
Regards,
Kim Siddorn