C9's at that vintage (2001) are HEUI pump engines. Those pumps can need replacing, CAT will have replacements readily available - suggest call CAT in your area and find out a price for two. They are not particularly cheap...
Also need to see service history on the aftercoolers on the C9s. Should be regular annual maintenance (flush with Rydlyme or similar) and new cores every 6 to 8 years.
HEUI pumps and aftercoolers are the known weak points on the C9s, there is lots posted on the web about that. If those are good, then that is an excellent start.
GB52 is a great boat, big and strong. Solid glass hull.
Hope it works out for you, it would be a fun boat to own.