this looks similar to the first racks i tried to build on modular grid!
i'm not sure what all of the modules are from the picture but I will give some general advice.
to start pick one effect module to process your external audio. put your audio into it and wiggle the knobs. listen to what happens and what you like. now look at the modulation inputs on the module. which of these inputs can do what you did with your fingers with control voltage?
modulation: start simply and go with doepfer. start with basic lfos and envelopes. listen to what these do to your sound. then add a mixer. mix your modulation and then listen to what it does to your sound. mordax data is very helpful for understanding what is going on but if you spend time listening it isn't completely necessary.
it will take a while for you to understand what it is you want from modular. I'm about two years into it, have definitely wasted a fair emount of money, and I'm now building a serge system. definitely spend time reading the manual before you buy something. it is boring but you will understand what the module does which is always helpful. youtube videos are good but remember more often than not another 5 or so modules are being used to create what you hear. they are not often a good representation of how the module you buy will sound when you're just starting.
it might be worth planning one effects chain and considering what it will do to your sound. if i were you i w0uld go for:
intellejel in/out, a filter, a reverb. after that you can add (modulation as mentioned above then) a delay. try delay into reverb. reverb into delay. move the filter to the end of the chain. etc etc.
I prefer effects modules that do one or two things well. you can browse by function here on modular grid so search for a reverb and sort by popular. there are good modules that aren't super popular but the popular ones are popular for a good reason.
as for the mono/stereo question. as far as i know most stereo modules will duplicate a mono input to both channels. if you want a stereo into mono i would put my stereo inputs into a mixer and take a mono mix out.









