Tough to give you a reasonable reply. The 3-53, 4-53 and 6V53 naturals are pretty tough. The 8-V53's perhaps less so. Most important is at what RPM the engines were usually run. If run at 2300 RPM they will be near dead already (assuming they turn up 2500 tops); if they were run at 1800 with regular oil changes, they will outlive most on Sam's Marine forum.
Keep in mind that commercial engines often go 20,000 hours or more with minor tune ups, like injector rebuilds. Downtime on recreational boats is tough on engines. 10,000 hours would not be unusual for a recreational engine. Even a superfifical engine survey by a good mechanic can detect major problems: engine sound, coolant condition, start characteristics, and operation under load should give reasonable feedback. An engine oil and transmission oil analysis is another cheap, worthwhile check. These are not finicky hi tech, hi HP engines that require perfection of each part.
