I changed my Tiptronic fluid (and filter) on my 2001 S a few months ago and it took around 5 liters. You'll be changing just the stuff in the transmission body, 3-4 liters of the old fluid will remain in the converter.
I used Pentosin ATF-1.
I bought a 5 liter bottle on Amazon:
Amazon.com: Pentosin (ATF1-5L) SAE 75W Transmission Fluid - 5 Liter: Automotive
I wasn't sure that would be enough, so I picked up a 1 liter bottle at my local O'Reilly Auto Parts (it's returnable if you don't open it).
You have to check the level with the transmission warmed up and the temp between 90-100F. Took me 4 check-and-top-offs (shifting through all the gears each time) to get it properly topped off.
I used the 101 Projects book as a guide. I think the pan bolts on mine were a different size than those listed in the book (T27 ?). I would check that you have the right sockets before you start, and make sure you pull the fill plug (17mm hex I believe) before you drain it (just in case). After draining there will still be a good bit of fluid left in the pan (close to a liter) so keep the pan level as you drop it. I used the amount that I drained as a reference for the fill. I also changed my drain plug (and seal) and fill plug o-ring.