If it was my choice to make, I’d rather drive a money pit Mercedes than a money pit Saab. Consider where the closest dealer is for each too. Your mechanic can keep either going with scrapyard parts in many instances with the Internet as your friend.
I didn’t understand the circumstances of this “family arrangement”. Certainly both cars can be expensive to repair. I don’t see the MB will be anymore to keep up than the Saab over then next 8 years. 8 years from now the MB’s book value will be significantly higher than the Saab’s as the Saab will depreciate more and faster.
This is a generous offer and it seems to benefit everyone.
80-90% of it has to do with how well it’s maintained.
I think that says it all.
I will only add a good independent who is knowledgeable about recent MB is what you need for more reasonable repair costs.
It’s all personal choice. I don’t like FWD and would take the MB over the SAAB. Both are more expensive to maintain and repair than typical Japanese cars, but they are nicer to drive. You get what you pay for.
Since you are married with a kid, remember the number one rule – keep the wife and kid happy and safe!
Twotone
You could probably make any car run longer or last almost forever depending upon how much care you take and money you spend. Ultimately rust will probably be the end of a vehicle . . . but even rust can be repaired. IMO the MB will do the next 8 years without much trouble . . . and so will the SAAB. 10 miles a day to work is only 50 miles a week, even if you double that for weekend driving, you’re only driving 100 miles a week, or 5200 miles a year X 8 years . . . and I’d buy either car with 100k or less if properly maintained. I would think that the only problem you’ll have is exhaust systems, since the car probably never really warms up enough to dry out the exhaust. Either way, I think the transmission issue nails it for you, as your wife may need to drive this car sometime. Rocketman
I think you will do ok by taking the MB, just make sure that if some major repair does come up later on you won’t have to take out a second mortgage to pay for it. If you can save say an extra 100 dollars a month to put in a repair fund that would be a good thing to do to help with future repairs or as an emergency fund. If you can put away a few grand for the future then you will be ready for most problems IF they occur. Keep up on the maintenance and change the fluids when they come due. You may be getting close to needing a brake job on the car and check the tires also.
I live way north of the MD line and I can ride my bike likely 90% of the time.
Take the Benz and have a happy home.
Eight years from now Saab may no longer be in business, while Mercedes definitely will be. The Mercedes will aso be worth a lot more; the Saab’s value will be close to ZERO! Your father did you a great favor; sell the SAAB.
Thanks to all you gave inputs. I have decided to take the MB. After struggling with the maintenance cost down the road, I figured money is not everything. The auto transmission, sun screen, and smooth ride is better for the family. After all, this is likely be the only time I would own a mercedes. 
Just around the decision was made, local MB dealer sent a email promotion for a B service for only $10. My dad thrww in a extra new pair of break pads. I have no complaints about this deal!
I would suggest 70 - 80% maintenance and 10% or maybe more, how it is driven.
Enjoy your new car. Stop back and let us know how it works out.
I know this is an old post but I can not resist to ask you… what planet are you coming from? Mercedes is not reliable nor it is Saab??.. while I would agree with your statement for the GM-SAAB cars as we the Americans’ managed to destroy this brand I can not agree for MB which is by far the best vehicle ever manufactured in series.
when you are saying expensive you thin only of money or also down time? Because while MB and Saab parts are more expensive they wear in a much longer time than japanese (one time use cars). MB is a reference brand in auto industry and Saab is well known for its safety. What are the Japanese known for? Economy… if I am not mistaking but even there I would rather drive a german/swede diesel car than a japcrap
What nonsense. MB, Saab, Volvo, and BMW are well known to need more repairs, and more expensive parts, than the ‘one time use’ Japanese cars.
Take your dad and mom out for a nice steak dinner and a good bottle of red wine. They deserve it.
I hope you realize how fortunate your are to have them.
Sincere best.