I really did mean to come back and write a why these parts post, so it wouldn't just be a big stack o parts, but stuff happened. But I'll do that now.
The Ryzen 5 1600:
Six cores, 12 threads, a bundled cooler that isn't garbage and the option of overclocking if you desire.
The I5 8400 may have a slight lead in single threaded performance, but it is only so if you obsess endleslly over numbers. Various spectre and meltdown mitigations have also eroded this lead somewhat. Should you put in the time and overclock, the ryzen will close the gap, while still giving the i5 a thrashing in multi threaded situations. From a price vs performance point, The ryzen destroys any intel offering.
MSI B350 TOMAHAWK:
It's black with shiny bits on it, contains your standard on board audio in the form of the Realtek ALC892 and networking also by Realtek. More importantly, all of the board's power delivery circuits are covered in heatsyncs. This will help should you venture into overclocking. The bios has enough options also, this board will push the ryzen 1600 to 3.7 GHZ and beyond if you're lucky.
Corsair Vengeance LPX ram:
I can't stress this enough, ryzen loves fast ram... so 16 GB of this stuff was a natural choice. There are no RGB lightbars, and the attached heat spreaders are low profile. If you ever decided to upgrade to a big dual tower CPU cooler down the line then this memory won't get in the way.
The Gigabyte GT 1030:
This graphics card is tiny, draws next to no power and can be installed by dropping it into a PCI express slot. If you need to add a GPU to a machine that doesn't have one, or has crappy integrated graphics then you get this one. It will also game if it needs to.
Now, for your extra questions
Reducing ram to get more storage out the gate?
I'd say only do this if you know that you will fill a 2 TB drive right away. A 16 GB kit of ram at that speed is not at all bad price wise and you really can never have enough of the stuff. The expandable nature of a desktop means that you can buy a couple of 2TB drives as you need them. Don't be that person that buys a 256 GB iPhone and only uses 68 GB of the built in storage.
What about the other processors?
The 2600X is more powerful, but costs more. B450 motherboards also cost a little more and you'll still need a graphics card.
You could use the b350 board, but it may need a bios update to support ryzen 2xxxx series CPUs.
The i5 8400 is currently hideously overpriced for what it is, even with the integrated graphics. Prices for something based around the i5 were coming out to much the same prices due to how overpriced it is currently coupled with the fact that you have to buy a third party cooler for the thing. A six core, six thread CPU should not cost that much, but there you have it.
The ryzen 5 2400g?
This is the one I have the biggest soft spot for honestly, 4 cores, 8 threads. Intel would have made you pay 2-3 times the price of this thing not so long ago for something similar. The integrated GPU is miles better than intels, It comes with the wraith stealth cooler (People have managed to overclock on this thing) and it still has plenty of power. If you don't feel that you'll need all the threads of the ryzen 1600, this is the one I'd recommend honestly. I could put another build up in post 19 later on, you can decide what tickles your fancy more between the two. I do honestly feel so bad for shitting on Intel so hard, but their stuff is just not worth it at the moment. They've been refreshing skylake for far, far too long now. To finish with, You thought your wall of text was bad?
All the seats are taken in the house that makes the rules.
All the seats are taken in the parliament of fools.