I have a Monarchy 18B DAC. It was old when I got it (maybe ten years ago) and it was outdated even before then but it's put in some very solid service and is the one piece of hi-fi equipment I have that hasn't needed repair (so it's durable) or mods (so it's well-designed). Really, old DACs are every bit as good now as they were at the time; there's just the risk of it not having borne the years well. Make sure you have a digital output on your computer or sound card first, of course.
Probably more than a good DAC you'll want a good pair of headphones and a headphone amplifier. A DAC, head amp, and pair of headphones is all you need for a nice system if that's all you want.
I've ripped everything to FLAC since 2004 or 2005 so I've never had to go back and redo them all. It's your call whether you want to do that with your extensive collection.
Buy two hard drives and back one up to the other every so often. You can get a 4TB one for only about a hundred on Amazon. You really, really, really don't want to lose your collection and hard drives, whatever their age, will die with little notice.