What is the purpose of this instrument?
What kind of sounds are you hoping to make, and for what style of music? This looks like a basic subtractive synth, and you could accomplish the same thing, with more features and less cost, using a semi-modular. You could integrate it with other cases later.
Are you buying all behringer just for cost reasons, or do you really love behringer that much? (not hating, I love behringer and don'tcare who knows it) 84hp isnt a lot of space and there are plenty of other low cost alternatives that wont eat up so much hp. It's good to budget, but you get what you pay for. (not a dig at behringers quality) Sure you're paying less but you also end up with less space, whereas you might pay a little more for a smaller module but you'll end up with more space to add things later on when budget isn't an issue and you figure out what you really need. You might want to add some flexibility to your budget to get flexibility in your case.
Do you specifically need a keystep pro for all 4/5 channels, or would a regular keystep work, after multiplying the cv and trig? It would save you money so you can buy more/better suited modules to acheive your goals. If you want to syncopate the osc you could use KS for ½ and the s&h circuit for the other ½.
I wouldn't plug headphones into your VCA for a variety of reasons. Mainly you could destroy the drivers, but also you'd likely only get sound on the left side since it's a mono output. Thats not as much of an issue for feeding your interface, but a dedicated output module will work better and save you a lot of trouble when trying to bridge the gap between eurorack and line level audio, but you need room for it in the case, and in your budget.
edit: I see you're using € instead of $, so I'm assuming you're in Europe somewhere, which opens up a lot of diy/built diy options to you depending on where you're located. I'm in the states, but have started ordering from pushermanproductions.com and thonk.co.uk for these reasons. Even with shipping, I'm getting more for less.