The 903's are especially sensitive to coolant/additive mixture, require a special blend to keep the liner O-rings in condition. If the mix is incorrect, or if straight anti-freeze is run, the liners will pit rapidly, and coolant will mix in the oil.
Test strips are available, but what a PITA!
These motors were designed to be tank motors, built heavy to stand up to radical abuse, but if you talk to some old gray bearded military mechanics, you will find that they weren't that great in that app either.
Sportfishermen around here say the 903 noise or vibration or whatever it is raises big marlin, don't know if that matters to you, but some owners baby these motors along just for that value.
Bottom line, if you are really in love with this boat, make your offer to reflect the need of an almost certain repower.