Some thoughts on this, in no particular order:
If you're spending appreciable money on building an always on appliance that you want to last for years, you want to focus on a solid platform, which in your case probably means a W680 chipset board, ECC RAM and a suitable CPU. You don't seem to have any particular need for a small build, so an ATX board makes sense as it gives you options for future expandability. W680 boards are not particularly mainstream, so look for a supplier for that first, and ideally some RAM that is on the manufacturer tested list. The obvious candidate (particularly since you've expressed a preference for Asus) would be the Pro WS W680-ACE that IceStorm mentioned. If you can, get the one with the bundled IPMI card, that is a nice feature to have for a system like this. This board has 2.5GBit ethernet onboard so it gives you the option to upgrade your LAN and have increased throughput.
"Suitable CPU" in this case realistically means "has integrated graphics and at least six cores". You could put a relatively low cost CPU like a 14400 or 14600 (both have six P cores but you get more E cores with the 14600) and it would still be considerable overkill for what you want. Set a reasonably low power limit on it (80W, say) and it'll run quiet with any half-decent cooler, but that's still enough power budget for plentiful performance. You absolutely can spend more on putting a 14700 or even a Xeon part in there, but for a box that is realistically going to spend 99% of its of time twiddling its tiny silicon thumbs, you're just increasing idle power for no gain in actual performance. I know you mentioned running VMs but the NAS part of this is, at most, going to wake up maybe two of the P-cores. That leaves you four P cores and all the E cores for....everything else. For reference, each E core is considerably quicker, per core, than your 2500k. So realistically, even the 14400 is overkill. You could probably do this just fine with the 14100 which is "only" four P-cores, but I can definitely see the argument for stepping up to the 14400 / 14600 (BTW don't get a CPU with a -F suffix, as these CPUs don't have a functional iGPU).
Two sticks of memory (Micron ECC should work fine - either 2x32GB or 2x48GB) and a couple of SSDs for a redundant boot and VM pool, and you're off to the races.
