Would be nice to know if it's a 5 cylinder 617 or 6 cylinder 603 engine. Most likely a 617.

AFAIK the 380SE wasn't sold in the US or Canada beyond the 1985 model year. IIRC there was no SWB 126 sold in the US or Canada in '86 or '87, then came the 300SE in '88. An easy way to tell an 81-85 126 from an 86-91 is the plastic cladding on the doors. Early cars have ribbed cladding, later cars have smooth cladding. Whether it's an 85 or 86 is far less relevant than the quality of the conversion and the results. An absolute hack conversion is unlikely given the abundance of 300SD parts cars. I'd look for Diesel features that don't work as expected (e.g., glow, tach, gauges), cobbled together fuel lines and back alley wire splices.

2.88 gears will be great if you do a lot of highway cruising and don't need to get a full load up a steep driveway.

Will Aircare be a problem on a conversion?

Sixto
87 300D