For an extra £25 you can get a Gen8 Microserver!
Now that's harder to answer, Plex forums might be your best bet there.
I would just be tempted to "overkill" and go for a i5-3470T (nearly twice as fast as the stock Celeron), not a powerful than a Xeon but less TDP and cheaper/easier to find.
I have a 1230v2 Xeon in my Gen8 and I've never had any issues, that said most of my clients "Direct Play" everything.
Why not create VM on your i7 and see how it copes with trans-coding DTS if you only give it dual cores etc? (to mimic a i3 3420)


Excellent, I just couldn't see how transcoding just the audio could be causing such a massive CPU hit, I was going to do some testing but you've obviously sorted it yourself.
Yep pretty sure, I can see from plex it's not transcoding the audio track and both the server and client app state "direct play".
I have a 2013 sammy that direct plays everything bar h.265.
h.265 is a total pita as the n54l cant keep up with the transcode. Haven't tried a file of that type since buying a T20 in it's place (but that's been rebooting every 10mins for the past week after only 3months of use, wheras the n54l gave me zero hassle. Waiting on Dell to swap the mobo).