you have most of teh main improvements already lined up so tahts a great starting point, if you are going to use the pc as storage/transport only then many tweaks associated with sound cards becoem irrelevant as you are outputting straight into the DAC and allowing that to work its magic, imho any prior processing of audio files (by the laptop) shoudl be kept to a minimum.
what you are trying to do here sounds very similar to using a server for content, i store all my content on a WHS without sound card and i only use the on board graphics card so i can see whats on teh screen for pc maintenace/operation purposes, it seems you will be using this laptop to control music too but you wont need fancy graphics cards to do this,
i think your limiting factor w.r.t sound quality is likely to be the interface from laptop to DAC, spdif is usually able to support a higher bitrate than many USB interfaces so id go with that if you can but not many laptops have this feature so you may be limited to USB.
All pc's benefit from a rebuild once in a while, i rebuild mine every 18 months due to the inefficinet ways windows works), the benefits here are around usability. when you rebuild partition the hard drives so you have OS on one logical drive and content on another, this gives you protection should you have a OS problem, you may also consider another partition for back up purposes, this is as fail safe as you can go with 1 physical hard drive (configured in 3 logical partitions)
hopefully that clears some of the fog 🙂