I flushed and refilled the system in my 560SEL when I changed mine, and changed the filter in the reservoir cap. I think I went through about 4 quarts, and used the fluid from one of the internet parts sites above. (partsshop or fastlane). It is insanely expensive from the dealer, $25 USD / qt, I think, and less than half that price here.
Your car may have a different capacity, as it is a different model, but I would guess it is close.
1989 560SEL (172k)
1989 325IC (122K)
2004 Suzuki Volusia LE (3500 miles)
2005 Yamaha Road Star (20 K miles)