I have a M50-powered 525 with an auto trans. I want to flush the fluid and give it a new filter. What is the proper ATF to buy? I'll probably go with a Redline ATF. Since without a pump to get all the fluid out of the torque converter, etc. how much fluid should I expect to refill?

If you're not connecting a machine to the cooling lines and getting *all* of the old fluid out of the torque-convertor, then you're not actually flushing it, you're simply draining the pan, and changing the filter.
The owner's manual on my 95 525 Touring says that for 525 and 530, just a change is 3.2 quarts. I'm very sure this does not include what's in the torque convertor.
It also says to use Dexron II on the 525/530.

Doubtful that it will drain the torque converter too. Definitely do the filter and pan gasket while you're there.

Should be D4 ATF but I would check to confirm.

i have heard Dexron and D4, are they basically the same thing... ?

Yup