I think first you should figure out what the sellers mean by "shot motor". Could just be a bad head gasket. Next, there is a big difference between a 300SD and 300SDL. The former uses an OM617 engine, the latter has an OM603. You can buy a good used 617 for probably under a grand. 603's tend to be 2-3 thousand used, unless you find a steal like the one on eBay this week for $1000 (no compression test though, and not in the car.)
Brand new (ok, MB factory rebuilts) 617's are something like $4k with all accessories, and new 603's are about $12k. MB does not offer a rebuilt 603. You can buy a rebuilt 603 frm Metric Motors for about $8k plus core and shipping.
Bottom line: Unless you have a source for a GOOD, cheap, 603 you could be taking on a very expensive project. If you are looking at an older SD with a 617 those are cheap & easy to find.
