Once we get into cars of this era, car valuation tends to be done on condition. So, for example, a dealer with a concours winning example on his forecourt might price the car at £24,000 - but this would be top money for one of the very best examples. A very good car sold privately (a Condition 1 car) might be up for around £16,000. A Condition 2 car (average) might be around £8000 while a Condition 3 car (rough, but could have an MoT) might set you back about £3000. It sounds like your car is in the middle to upper tier of values for this model. If you're not sure have a look around for recent examples sold at auction, in the private ads or on eBay and see what sort of money they fetched. Values for the R107-series Mercedes SL have risen pretty steeply in the past ten years. Two owners with full history suggest a car that's been well looked after. So why hasn't the oil leak been sorted? These models do tend to leak a bit from the cambox, but there's also a variety of other places they can leak from, so more investigation is needed. How much oil is it using? How long has it been like that? What has the owner tried to do to fix it? If you're not a hands-on person, arrange with a local garage to take the car in for an inspection. And if you need an expert on hand why not try the Mercedes-Benz Club UK? They may have a knowledgeable member living locally who might be able to help you assess the problem.