10-year-old German luxury sedans - Mercedes, Audi, and BMW - are very tempting because they are very inexpensive for the level of luxury you get.  But the reason they have depreciated so much is that they usually have a lot of very expensive problems, so that the total cost of ownership is actually not that cheap.

3rd party "warranties" don't solve the problem.  They aren't like collision insurance that is intended to cover unexpected catastrophic damage.  The warranties are really just a pre-paid plan to fix the normal, expected things that go wrong as cars age.  But the problem is that they are priced to cover the expected repairs plus a significant markup.  The warranty company doesn't fix anything, they just take a cut and pay someone else to fix the thing.  And the way they make money is that they are very good at not paying for things.  And with a German luxury car, they will be motivated to not pay for a lot of things.

But there is one way you can turn the situation to your advantage.  The sale price on a 10-year-old S class is low because of the expected repair costs.  But if you have a way to repair things cheaply, you could come out ahead.  3 ways to do that:

1. Do it yourself.  Do you have the tools, experience, and motivation to fix most things yourself?
2. Make friends with a good independent German-car mechanic who will work for a reasonable charge.  If you have a good one in your neighborhood, that might work.  Unfortunately, I suspect most such mechanics are used to working with a wealthy clientele and are unlikely to work for cheap.
3. Take it to a good shop in Mexico.  There are some.  Not necessarily many, but some.

Otherwise, I'd look for some other type of car.

Cheers!

If you're concerned about the cost of ownership, a Mercedes isn't for you at all. 3rd party warranties aren't worth the paper they're printed on if you read the fine print. They're toys for rich men, and when they break, they cost an absolute fortune to fix, and they will break. An every once in a while car that lives in the garage most of the time? Maybe. Looking for a daily driver while on a budget? Way no. 

The possibility of turning into an endless money pit is strong with this one. The vast majority of aftermarket warranties are worthless. The glowing promises they make in their ads and brochures are taken away in the fine print.

Posted by: @bronxbomber02

I was thinking of buying a 2014 Benz S550 what aftermarket warranty would you recommend??

Why? You have a need for 2X the pain?