I try to keep new files as flac, but honestly flac vs 320mp3 has next to nothing audible difference
First - Congrats on a smart choices!
Getting a dac probably is not good investment right now, in fact you'd be probably better served by a used surround receiver - I can bet you can find a decent used one for about $50-60 ( make sure its fully works , has at least one digital input and supports DTS) This will help you especially since it will help you integrate a Subwoofer. Only con is avr typically big and bulky