I'm in a similar position now but the prices on the r350 and r320 are in the $26-$30k range for under 40,00 miles. I really like the styling of the cars and room, but I am very worried about reliability. I am coming from a 199 Infiniti QX4 which I have driven 160k miles replacing only the distributor and air compressor (I did replace a timing belt as a preventative).

I'm between the Mercedes and GMC Acadias which on average is a couple of grand more than the Mercedes. Aside from the better gas mileage (not so much over the diesel), I have never owned a German car before and the Acadia I am sure will be chaper maintenace wise. I am most concerned about reliability, I don't want my wife to be on a 4 hour trip to see the In-Laws and break down. I have been out of the car market so long I have no clue about vehicle reliability for most anything out there. Toyotas and Hondas still have the reputations, and while that is the extent of my "knowledge" the Acadia just seems like the vehicle that might be more reliable.

I'm not opposed to an extended warranty, any idea what one would cost, and do MB dealers typically offer/sell them for cars purchased elsewhere (internet)?

I don't know if this helps, but I bought a 2005 E320CDI with 107000 miles from a M-B dealer in 2010: it has 150,000 miles now, trouble-free. It uses no oil between changes (12,000 mile interval) and I am well pleased with it. Paid $15,499.

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Providing a home for these cars:
1951 Buick Special De Luxe
1977 Rolls-Royce Silver Wraith II
1970 Rolls-Royce Silver Shadow
1986 Mercedes-Benz 420 SEL
2005 Mercedes-Benz E 320 CDI
2012 GMC Sierra 1500 4X4
1927 Pierce-Arrow Series 80
1931 Pierce-Arrow Series 43
1926 Ford Model T coupe