how many miles on orig motor and trans

Mine hasn't died yet. What vehicle are you asking about? WRX, STi, RS, TS, L?

I've heard of Subaru DL's with over 300,000 and still going.

It's something that is a variable based on operating conditions and driver.
My '98 RS had 100,000 on it when I sold it without any major engine work and it was still going very strong.