Mercruiser raw-water heebeejeebees

[svtosca]

On the verge of buying, but raw water cooled engine -- though clean as clean can be with full service records, etc. -- scares me. Can anyone tell me how many hours (or years), realistically, you can expect from a professionally maintained and meticulously serviced, raw-water cooled Mercruiser I/O? Or better yet, any long-livin' raw water cooled Mercruisers out there? What's a conversion to closed cooling go for on a 5.7L?

Thanks.