I've been wondering how to breach .hack in general. I know I have the sign anime series and the original quadrilogy, but I'm trying to see how I should get to this as I managed to pick it up on the cheap.

I checked out a bit of the first series and the mixed reality stuff was a REALLY cool thing I wanna get deeper into.

The gameplay doesn't hold up too well, so I can't really recommend it. Last Recode comes with a recap of the story, which is good enough. I'd consider checking out .hack//Roots anime (or at least the first half of it) that serves as an introduction to G.U. more than I would recommend trying to play the original games.

It makes references to those games but you don't need to get those references to understand the story.

GU is pretty standalone (not counting Roots which is an interstitial piece that GU leans on but doesn't require), but you'll miss some things that are put in place for people who played IMOQ. For example, the main antagonist of GU Vol. 1 looks like the main character from IMOQ, which means nothing to people who haven't played those games but throws people who have for a loop. Then there are some characters who were characters in IMOQ (but this also doesn't matter, it's for nerd appreciation).
The IMOQ games are pretty pricey nowadays due to their rarity; I got lucky and got all four used in a lot auction on eBay for $150 USD many years back, but Quarantine by itself will probably run you $120 or so. My recommendation is to watch a Let's Play on Youtube; it's cheaper and those games play like trash. NakaTeleeli did a great job with IMOQ in my opinion.