Hey,
Like Neil said, you'll probably want a A119 module.
There's also the new A138d that will do the impedance matching stuff
if you want to use effects that are not in the A100 (stompboxes etc).
Choose a set of CV sources (theremin, LFO (VCLFO for the win !!)...)
Take a look at the A188 BBD modules, great delays.
Consider buying a VCO that you could use for frequency mod or with a
ring mod.
If you plan to do substractive synthesis (filters) then consider using
a compressor or something that will nuke you guitar signal so that you
don't end with huge attacks and no sustain at all (for example when
you use filter with envelope follower).

I personally suggest, althought it requires non Doepfer stuff, the
A106 Xtreme filter + Metasonix TM7 scrotum smasher. If you like dirty
trash extreme sounds, that's the most incredible combo I've eard.
Something between a pig screaming and a nuclear blast.

So,
-Inputs/Outputs modules (119, 138d)
-CV modules : follow your instinct, pick the ones that you can use
while playing guitar (joystick is great but you need a hand to use it
heh !)
-sound processing modules...

There have been a few discussions on this list about guitar and A100,
maybe you can try to find them out to get cool ideas.

Hope that helps,
Max.