Hey,
A couple of remarks:
The selected GPU is very week and cannot handle 4k transcoding.
If you are going for Media Center you need a lot more storage (you can get significant more storage with HDDs compared to SSDs for the same price).
You should probably look at an Intel CPU (their CPUs are shit for gaming but man, Intel QuickSync on the on-board GPU is one heck of a power house for transcoding). This way you don't need a dedicated GPU and can still transcode.
Your currently selected Motherboard has a Realtek NIC, Realtek NICs are mostly not supported and/or are not working correctly with Hexos/Truenas so if you choose a MoBo with a Realtek NIC you also need another (preferably Intel NIC) to get any network connections. In combination with the HBA card and a dedicated GPU (if you are going that path) you need to make sure to have enough PCIe slots and PCIe Lanes for those 3 extansion cards to work properly.