The Mercedes recommended fuel for the E280 is 91-octane regular unleaded, while the recommended fuel for the C180 Kompressor is 95- octane premium. While you can safely run the E280 on regular unleaded, and I suspect you could run the Kompressor on 91-ocatane, you will get better performance and economy by running them on the higher-octane fuel. Mercedes-Benz also approves the use of E10 ethanol blend fuel in both of your models, which might help you save a few pennies.

You've obviously done quite a bit of homework, and it seems you have done the most important thing: identify what it is you want from your toy. As your regular cars are prestige cars and not dissimilar to the Jaguar model you are considering, you could consider a sportier car, such as a coupe or a convertible Mercedes or Saab, which would be a little more special, or perhaps even a Jaguar XJS. Any Jaguar is quite a good choice, though the Series III suffered because Jaguar were too cash-strapped to de-bug it properly before it went into production. That and shoddy build quality mean many of them gave their owners heartburn. But there are many around that have been sorted and they make nice cars. There are several Jaguar specialists who can help find a car for you, but remember they are in business to make a profit. Caulfield Jaguar rebuilds Series III XJ6s to your specification. They appear to turn out good work, though their rebuilt cars will cost more than the $14,000 you want to spend. I recommend you contact the Jaguar Car Club and get the advice of people who have owned the models you are interested in buying. Their experience is invaluable and real.