Hi Steve. Welcome to the group.
Pre-purchase inspections are critical. If at all possible, it should be done by someone who knows the Pagoda well. Also, keep an eye out for rust, and learn where to look for it. Unfortunately these cars can rust from the inside out, and trouble might not be readily apparent.
Additionally, I think you wouldn't regret taking the necessary up front time to get to know the vehicle as an observer first before taking the plunge. That way you can spot irregularities quickly and know when to walk away and when to pounce.
On the particular car you noted by link, it appears to have non-spec interior and carpet pieces as well as exterior rear view mirrors for instance. That might not jump out to someone who is new to the series, but might pop right out to someone who has looked at them more.
The one I own is a 230SL and there are many things about it that I wished I would have known before buying. Thankfully, many such things can be learned by scouring the sl113.org website. The latter is a much less expensive proposition than the former. This site is a goldmine of information about these cars, and even more, is a community of people who enjoy taking care of them. These guys are great and are willing to help.
Glad you're here and asking for advice as you have is a great place to start.
Clay