Hi NCBug,
A lot has been said on that subject (too much sometimes...) in this forum, do a search for 915 or so.
In short: As of '77/'78 ?(3.0 SC engines) the cases were alu as they were magnesium before.
Also, I believe LSD was option or standard on some versions. If you want that, now is a good time to look for them.
IMHO, the most important thing to look for, besides the tranny being as in good a condition as possible, is the diameter of the output drive shafts.
Most are 110mm, which makes it impossible to use VW splined axles.
Some (early?) versions have 100mm output CV flanges, which make it very easy to bolt it to a (Bus) 100mm VW CV and subsequent axle.
I use a 915/40 trans from 1976 (2,7ltr model) which has 100mm output flanges
Greetings,
Walter