This is something that you can do yourself. There's plenty of information online - you can even check out our bullet point buyer's guide on the R129 series here: classics.honestjohn.co.uk/reviews/mercedes-benz/sl...g
But if you're not confident or comfortable inspecting the car, then you should definitely take back up. Have a chat with the Mercedes Benz club (mercedes-benz-club.co.uk/) and see if they have any members local to the car who would be willing to help out. Failing that you can use a company like the RAC or even get the car taken to a local garage for a multi-point inspection - you won't get the benefit of having a specialist going over the car though, so might not be aware of some of its more intricate foibles.

And remember that these inspections are only good on the day - for example, the car might not show signs of overheating during the inspection, but on a 3hr journey in heat could develop some issues. Whatever you do, make sure you see and drive as many examples as possible so you have something to compare this one to ... it's never a great shout to buy the first you see (although we've all done it). If you're not mechanically minded and looking after the car is something that concerns you then consider buying for a specialist dealer (such as The SL Shop) which specialises in SLs and carries out pre-purchase inspections and offers some kind of aftercare package.