Do you care about resale value? If yes, avoid Mercs
Are you mechanically inclined? If no, avoid Mercs
Do you have an OBD scanner? If no, avoid Mercs
Do you have male and female torx bits? If no, avoid Mercs
Now if you answered the opposite to all of the above, then getting a used Mercedes is alright.
If you perform the maintenance yourself, and treat it right (don't skimp on higher/lower viscosity oil), then the car treats you well back. There is a large enough aftermarket support (Bosch, LiquiMoly etc.) to save a few bucks without sacrificing quality. Of course aftermarket parts will dent the resale value.
Taking an out-of-service plan Merc to do the dealership for annual maintenance or service is financial suicide. A diagnostic, report aka a detailed OBD readout on their star system is ~R630, then a post service diagnostic report which shows the error code has been cleared is another ~R630. Then the actual service will charge labour at R900 per hour, excl parts. The silver lining is that you get free coffee, sometimes live music, and chauffeur to your work + back to the dealership.
The A class (W176) you mentioned isn't so fitting if you want luggage space for 2x children. Might want to consider a B class if you don't mind the soccer mom stigma of driving a MPV.
Lastly, avoid anything that has an airmatic system in a used Merc (S, ML, CLS, GLE, GLS, and new C-classes). I've dealt with those and they are hemorrhoid-inducing to fix.
Honestly though I don't think a mother of 2 has enough time to fiddle around the mechanics of a car, she is better off getting a in-warranty+service plan Honda Ballade 1.5 manual (I personally don't trust CVTs yet).
