Only buy from someone you know and along with it the person actually maintains the cars properly .. otherwise not a good buy as I think.
Among merc/bmw, merc parts would be more readily available and its resale is comparatively better. But if I was you i would not buy those german cars as i have seen many which the owners claim to be well maintained but there are so many jugars in the cars.
I know someone who got a merc 2007 and its head-light stopped working at one day. And he had to pay 45k to get it fixed as its blaster was also dead. Blaster failure not comes in bad maintenance but the cost of it was too much for a 2007 car
Then I know a person who got a audi 2015 A6 for 75 lacs and was happy he made a good deal but now that car is standing in a workshop for last 3 months and there is some error in computer. Now that guy knows if he sends the car to audi they will give an estimate of 8-10lacs and they are trying their best to get things fixed and bring the car back to road. This is how people normally maintain these cars in pakistan as the maintenance is expensive and people who buy 0 meter actually do keep them maintain but once its in second user's hands just know its not properly maintained unless u know someone personally.
I myself stay away from these cars although merc being always a dream but i will buy when either i can buy brand new or i know 2 people in my circle who have mercs/bmws 0 meter bought and driven like 10-15k so far in 3-4 years so will wait for those 
For the price of 2007/2008 germans why don't u buy a toyota suv. Although it will be old and khuwari as well but that khuwari will be one time and if u wisely spend on its initial maintenance once it will run longer and the cost of maintenance if not cheaper will be same as those of merc/bmws but everything is available easily and better resale as well. Just what i do myself