Sound Card Interface
If it was me, I'd interface the digital modes via the ACC connector (13 pin DIN). That way you can leave the Mic connected and not have to re-wire to change modes.
As for an audio interface, let's see what Google offers up: http://www.wraith.sf.ca.us/electronics/radio/rigif.html
Looks like you're on the right track, see how they sum the two audio channels into one using 2 - 2.2K ohm resistors.
See the comment to keep the grounds isolated. Of course, the rig control interface will connect the grounds, so I think that may be less critical -- I seem to recall just using pots (variable resistors) for level control. You'd need opto-isolators on the CI/V control side to maintain complete ground isolation, I believe -- I don't have the schematics for the radio here.
After a while, I wound up using this device (SignalLink), as it contains a built in sound-card and VOX-operated transmit. Made things very easy and allowed me to maintain the speaker connection at the PC.
http://www.tigertronics.com/slusbmain.htm -- $110.00 + ship with all cables, etc. needed. (The CI/V rig control is not a part of this scheme.) It worked really well and looked great.
That way I didn't have to futz with cables to use the PC for different purposes. And I personally like the esthetic of a really tidy station. YMMV, of course.
In this vein, here's how G4ILO built his own: http://www.g4ilo.com/usblink.html
(BTW: He claims that most modern sound cards have the transformers built-in so adding your own is redundant and un-necessary. So check.)
The Voice Operated Transmit (VOX) circuit to key the PTT line on the radio:
Hmmm... I'd love to get a 2M WSJT meteor scatter station w/ a beam on the roof Martin. --http://physics.princeton.edu/pulsar/K1JT/ -- just keep that in the back of your mind.
Of course, once we get the radio and PC interfaced, running WSPR will be trivial, esp. if the PC can boot into Windows: http://wsprnet.org/drupal/