Since you like JRiver, and also use MPC, I'm going to assume that you are using LAV and madVR enhancements. For those, you need a bit more horsepower, so I kept that in mind as I looked at options.
Are you using HDMI audio, or separate audio out from the motherboard? I'm guessing HDMI audio since you didn't mention audio earlier.
Just checked out the HDPlex website, and I see that the H1.S (sweet case, btw!) CPU heatsink system supports a bunch of intel sockets, plus "
APU with AM4/AM2/AM3/FM1/FM2 socket".
I know you said you prefer intel, but since you are building a dedicated HTPC, I think it is worth considering a modern AMD APU on the AM4 platform. AMD's graphics are superior to Intel, unless you want the Intel SGX feature which AMD doesn't offer. I don't know if you would ever plug in a Blu-ray drive via USB and want SGX, but I'm assuming that is not something you would do, so having SGX support is pretty much pointless.
The AMD Ryzen 5 3400G is a Quad-Core 3.7 GHz base / 4.2 GHz Max Boost 65W CPU, and it has built-in Radeon RX Vega 11 graphics. It's only $150, and that includes the cooler which you don't need, and you could sell that on Ebay for $10-20 if you wanted. The Radeon RX Vega 11 will also handle the HDMI audio out, so again it's a solid choice.
https://www.newegg.com/amd-ryzen-3-3200 ... 6819113571
That Ryzen 5 3400G is faster than the i3-8100 in pretty much every test, and has immensely more powerful graphics.
Plenty of motherboard options too. Like you, I like ASRock motherboards (I think everything in the house right now is ASRock).
The ASRock A320M-ITX AM4 is a nice budget option at $95, and will probably do everything you need for HTPC duty.
https://www.newegg.com/p/N82E1681315783 ... -_-Product
If you want the newer AMD 4xx series chipset, for $170 you can get the ASRock Fatal1ty X470 Gaming-ITX/ac AM4. Seems like overkill for non-gaming, non-overclocking HTPC purposes. But it does have 1 Display Port and 1 HDMI, where-as the A320 above has 2 HDMI.
https://www.newegg.com/asrock-fatal1ty- ... -_-Product
Both motherboards above have the Realtek ALC1220 audio chipset with 7.1 output, which is a great audio solution (if you're not using HDMI audio). The A320 should run with lower power and heat, as the chipset is very basic and power efficient - perfect for an ITX in a HTPC.
Regarding memory, I would always pick that last, and pick something from the motherboard's QVL to make sure it has been tested to work with that motherboard - especially if you go the AMD route, as it is a little more finicky on memory. 16GB in a 2x8GB configuration sounds good.
A 64GB drive should be plenty for Windows + HTPC software. If you are planning on storing videos or recording to the HTPC, then definitely step up to a larger drive - and for that even 512GB sounds kind of small for recordings. That same drive in a 1TB for $95 is a sweet deal. But if all you will be keeping on the drive is Windows and HTPC software, your 64GB drive should be fine (unless it is really a slow M2 SSD and not a faster M2 NVMe drive - small distinction, but big difference in performance).
Ultimately, I don't think you could go wrong with either path, the i3-8100 you picked out, or the AMD Ryzen 5 3400G above. If I was chasing SGX functionality, I would go the Intel route, but if you have no plans to play physical 4K discs via a USB Blu-ray drive, then the superior graphics horsepower of the AMD gets my vote.