the Audio Interface module does not replace your computer audio interface(s), it's just a way of amplifying line level to modular level and attenuating modular level to line level - you may or may not need attenuation at all - a lot of audio interfaces and mixers can easily handle modular levels on the way in...
the audio interface wont really help with pedals, either, instrument level is even lower and a different impedance - and you've got the guitar interface already - better to get a veils as it can amplify line level to modular level, especially if you don't need it all the time - as for guitar pedals - get a proper interface for those
I wouldn't worry too much about 1/4"/1/8" there are plenty of cables available that do that... these are mostly what I use for this
as you are a guitarist, I'd also look at some modules to enable you to use your feet to control the modular - expression pedal and foot switch interfaces - addac or doepfer, for instance
I wouldn't suggest another vco - you have 3 already... that's enough for this amount of hp imo... see my signature for some useful advice on how to get the most versatility for the least cash...
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities