Mercs are actually a bit shit by the way (my one-woman crusade). People think they are great but they aren't really. We also have a few friends who have "saved up" and bought them as dream cars, once they got financially secure etc., only to find they were rubbish.
Age isn't necessarily the gauge. Manufacturer is.
Mercs were good, then they went shit, now they're getting better (or are better) again. Good manufacturers can make bad models (Lancia Beta, anyone?), sometimes there can be a few bad examples of a particular model.
Most Japanaese cars like ... Toyota etc. are really well made and engineered.
How quickly we forget the massive issues with sticking accelerators and the huge recalls.
Buy on condition, not age. A well-looked after car will run for a long time (especially if it doesn't have complex electrical systems such as in newer cars). This is why 50 or 60 year old cars (and older) are still running today and keep on going - they are mechanically fairly simple and are cared for by enthusiasts.
Buy a new car if you can afford it, and by that I mean the huge depreciation on new cars. Factor in resale value against purchase price and consider the total cost of ownership. A quick look at 3 or 4 year old Ford Focuses suggests the depreciation is in the region of £10K+. If you bought something at around £5K, assume depreciation is £2K over 3 years, you've got £8K to spend on repairs before you even match the money you've lost on a new car.
If you've got the cash to splash and must have the latest and greatest, go new; if you don't want to look after your complex, painstakingly-designed and engineered car capable of travelling over 100mph, prepare to pay someone else lots of money to maintain it, or lose loads of money when it dies due to a lack of mechanical sympathy.