If the 617 engine car is running 20 mpg, it possible is running a little rich. I would say 24 or 25 miles is about right. Some people reported 28 mpg.
I will not worry about the tranny different between 84 and 85 since the pan gasket is readily available and cheap. The same pan gasket for the 85 is used on other models too.
The major different between 84 SD and 85 SD is the Catalyst (or trap oxidizer before the recall)in California. It is expenive to replace. If you are one of the unlucky one who needs another Catalyst after one year, it is about $800, part and labor. Also, 85 Calif SD was slowest among all SD due to the trap oxidizer. I do not have data to tell you how fast the 85 SD is with the new Catalyst. I have 300D and 300SD, both 85 Calif, one with the Catalyst and the other is still having the old T.O. As far as the performance concerns, I cannot tell the difference between them. I could modify the turbo to make the turbo kicking in early (about 1500 rmp instead of ~2500 rpm) but I have never tried to do so. The performance right now is acceptable to me and I do not want to put the extra load on the turbo.
SD is comfortable. I think the 84 has no memory seat but 85 does and the aux fan for the AC is larger in 85. 85 SD has ABS but not 84. 85 uses the 9-blade plastic fan and the 84 does not. The plastic fan is better. There are not other differences I can think of now; you already know them all.
As far as California models go, both 84 and 85 have an electronic module (as I call it computer). It is a very sample logic measuring rpm, tempurature, etc. to control the EGR and the turbo operation.
If every thing being equal, I will take the 84. However, price and cosmetic condition are very important to me for deciding which diesel to buy.