storage options for MacBook Air

I also use the AIFF format. And iTunes.

So the answer depends on the number of albums/songs in your library and if you plan to add to it.

I assumed that the external drive was used because you were running out of disk space on the MB Air. Which, as your library grows, is gonna happen.

For example, my Rollings Stones "Let It Bleed" CD to AIFF rip is 455 Megabytes. Not sure how much overhead Mac OS (or OSX) uses on that MB Air. But if an album runs 450 Megabytes per CD you could calculate how much of your library it could hold. 

If you like iTunes (and I do though I use Roon for playback but hear no difference in SQ from iTunes) then maybe consider a MacMini to run iTunes and output to the Ultra Fi. You control the Mini iTunes from the MB Air using Screen Share. That existing external drive could be connected to the Mini. 

Without knowing the type of player you plan to use it's tricky to suggest anything. 

Seems to me that the portable benefits of the MB Air are lost when it gets connected to a wired up external drive and DAC. 

Having you considered a streamer w/ internal storage? One that you could control with your MB Air or a phone/tablet?

I listen to music on a (mostly headless) MacMini. The music is managed by iTunes. I've got almost 3/4 TB on a 2TB WD drive connected via firewire to the Mini.  The Mini USB out to an LH Labs DAC to HiFi.  This is my Roon core. 

I rip and control the MacMini from an iMac which is a Roon remote. The iMac has an Oppo HA-2 DAC/headphone amp for local listening

Both are wired via ethernet and I use Apple's Screen Sharing to manage the Mac, add music files etc.  

Your MB Air is a sweet portable device, an excellent remote control to the hifi stereo but capable of playback to personal listening too. 

+1 for BitPerfect