Inexpensive, Reliable saden for bad roads

If bad roads means low traction then avoid rwd like the c class. C class are also expensive to repair and those years are problem bound.

If bad roads mean potholes the look for the smallest wheel size with high profile tires. This will limit the risk of bending rims. For best comfort look for a larger car to help smooth the road roughness.

I'd look at a Chevy Impala. For $6000 you can get a 09-10 year. With the 3.9 v6. This car gets over 30 mpg on the highway, although much lower around town.

If you want better fuel economy and reliable, you can a 2010 Prius for just over $6000. Look for the III or IV model

Not sure of the prices now but the early 2000's is when MB offered 4matic on the C-class. I dis not particularly care for it in none loaner I had as it vibrated the steering wheel/front of the car unlike the rear wheel drive.

Does the Avalon offer AWD, or is it like the Impala just fwd?

Not sure if pre-2009 Buick LaCrosse offer AWD and in your n price range, but that era LaCrosse has beaten the Lexus ES350 in comparisons.