The piston coolers are small jets placed into the main journals of the block. Holes were drilled in the upper (block) side of where the crankshaft rides in the block, and these jets where placed in these holes. They were fed by the grooves in the main bearing shells. You may not be able to see or feel them while the crankshaft is still mounted in the block; it's pretty tight in there. The same set-up was used by BMW in the '90s (at the least) on their six-cylinders (and maybe others), if you're familiar with those engines. If you're looking for some sort of auxillary oil rail or dippers on the connecting rods, forget it. MB's (and, by extension, BMW's) system is elegant, subtle and effective.
As to the pistons, the differences between the turbo and non-turbo engines were not confined to compression ratio alone: On the turbos, the rings were thinner and the steel insert in the piston crown were changed to allow for a different piston expansion rate within the cylinder bore. The piston skirts were altered, too.
Pistons are usually available at www.autohausaz.com. As you can see, they are currently on backorder, so, if you can find them for $200 each, you probably shouldn't hesitate:
http://www.autohausaz.com/search/product.aspx?sid=dqxijo55wnil3r45iqjlyd3n&makeid=800016@Mercedes&modelid=1193992@300SD&year=1980&cid=20@Engine%20Parts,%20Seals%20%26%20Gaskets&gid=5622@Piston
Keep in mind that MB didn't simply slap a turbo on their already legendary 5-pot diesel; they engineered an engine not only worthy of their reputation, but enhanced it as well....
Mark in Lakewood, CO
To: diesel_mercedes@yahoogroups.com
From: duckn8r@me.com
Date: Wed, 13 Apr 2011 16:51:48 -0400
Subject: Re: [diesel_mercedes] Re: Doing some R&D need your help
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