Sell the Mercedes on Craigslist for $1000 or whatever first offer he gets. He will need to have it towed home unless the mechanic will buy it. Spending nearly 5k on a 10 year old car is a big nope.
Search Craigslist for something like a 10 year old Toyota Yaris, Nissan Versa, Mazda 2, Chevy Spark, Kia Rio, Hyundai Accent, etc.
The best deals are wrecked cars with damage that is purely cosmetic. You get the same reliability for $1-2k less, and a completely different depreciation curve. i.e. when grandpa hits a parked car and the kids finally talk him out of driving, you buy his low mileage $3500 car with $2000 in body damage for $1500. Also, rust bucket cars are cheap. If the floor's not falling through, the damage is only cosmetic.
Avoid European brands (VW, BMW, Mercedes, Fiat) due to astronomical repair bills.
Avoid Fords (Mercury, Saab, Lincoln) because the transmissions always go out.
Avoid Chryslers because... well... lots of reasons.
Note that GMs have lots of electrical problems, but also depreciate quickly enough to make a used GM worth the hassle.