To get external audio into your Eurorack case, it's best to have something to boost the levels to synth level. There are several modules with this amplifying feature. Most manufacturers with a full product line offer a product that will do the job: Intellijel, Roland, etc.
My general VCA is an Intellijel Quad VCA (two of them actually). The module is very flexible and offers a lot of control over shaping of the CV inputs. It also functions as a mono mixer and has a boost feature on every VCA if you need a bit more volume or want a little overdrive.
My path through modular was this in a nutshell:
Download a copy of VCV rack and begin getting familiar with the nuts and bolts of patching the basic modules.
Put together my first rack on modular grid.
Go through SEVERAL virtual set-ups, tweak, research (lots of Youtube videos and manual reading), tweak again, start over, tweak, read... etc.
Break down two cases worth of modules I'd like to one case of modules that would work as a starting point.
Hold breath and order first round of modules. Then order additional modules planned for case one. Between each order I played with what I had for some hands-on experience and then made a qualitative judgment if things were going to plan. I adjusted what to order next based on observations with previous order and some assumptions/errors that I made.
I bought an Expert Sleepers FH2 early on. I'm not sure if it was a good decision. I found trying to flow between Ableton and Eurorack to be a bit clunky. The FH2 has to be configured before loading Ableton. So if you want to change the set-up as you're jamming, you have to stop, quit out of everything, change, then reload. If you never change your set-up then you're good to go. But if you're trying to take advantage of envelopes, LFOs, etc in the FH2... it's rather clunky. I found keeping everything inside of the case to be better for my workflow. I'd hope to save a few dollars by relying on the DAW... but I just didn't like it. I still have the FH2 as the firmware is constantly being upgraded.
The biggest compromise is budget. There's a ton of stuff I want...there's only budget enough for a fraction of it. :)
This is my current set-up. There's a Pittsburgh microsequencer that I pulled out because I wasn't using it and didn't like it. There is also a Behringer Neutron and Behringer Model D that float as they don't need to be in the rack.
I'll probably add a third case within the next year. The 2HP reverb and delay are okay. But I want stronger modules with more modulation options. I'd like a 1010 Music BitBox, more modulation sources including a Zulu trigger sequencer, some switches, faders, a pair of matching Dixie II+ oscillators, and possibly another full sequencer as the Toolbox is great but not great for chaining sequences into songs.