I'm not a tech, but from what I have read on this site for the last 7-8 months, mercedes bottom ends are usually very reliable.
I haven't read a story at least I don't think I have yet about a bottom end blowing out. Valve stems and seals, head gasket, timing, etc, are all that I read about.
Any other brand of car, I'd agree the bottom could possibly go out, but then again, it could last for a 100k or more befoe it does.
If your shop is good, and you are hapy with their work, and they are good at what they do, you will end up happy.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.