Yea, it's disappointing. I was hoping things would be good enough that I'd get away with a ring job, but I wasn't banking on it. I guess what I do now depends on how the cylinders and the other pistons check out. If the other pistons aren't too badly worn and the cylinders are close enough to do a hone and go I'll definitely be interested in picking up a piston. I am wondering what the best step will be if the cylinder walls are too far out. The piston set listed on the parts section are a 90.9mm, which I assume is means an overbore size. Is overboring a good way to go, ie, better than a resleve if the bores are out but the pistons are ok? I guess if the sleves and pistons are bad it's either bore and install oversize pistons or do the whole job with standard sizes; doesn't seem worth spending an extra $300 or so on the sleves if you can do basically the same thing with oversize pistons. Just thinking out loud, I guess.
Anyway, the machine shop I'm planning on taking it to was referred to me by a shop that does a lot of diesel work, and they seem somewhat familiar with Mercedes diesels. I'll be back if there are any new developments, thanks to all for the advice.

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Once and future king:
'64 Ford Fairlane w/approx 238,000 - looks rough, but amazingly reliable if you know how to look after it; I will soon begin work to totally restore and modernize it.
Family vehicles that I lay some claim to:
'78 300D w/approx 350,000 original, '62 Ford F100 4x4, '90 Ford E150 w/171,000 original