Yeah it certainly can be time consuming finding good mods for AC, and you're right, the lack of any kind of centralisation is what makes it hard. I disagree with the "stick to Race Department" advice people often give newbies though, as although people act like it's got the best quality mods, the truth is about half the AC mods there are really poorly done rips from RF1 and 2, and I've yet to find a good quality mod there that I can't find anywhere else. So while I do use RD, it'd be very bad advice for me to tell anyone to stick to RD for quality mods.
If I had to give a recommendation for newcomers it'd be this: Join RD, join several FB mod groups (some good ones are Assetto Corsa Mod Central 24/7, Wack Mods for Assetto Corsa, Assetto Corsa Warehouse, and NaVi Street Cars), and join/bookmark other good AC mod sources like F1Classic, assettocorsa.club, assetto-DB, AC Drifting Pro, and AC Track Reboot. I'd also tell them to bookmark SimTRAXX and consider joining or donating to them for some of their excellent tracks as well, and I'd lastly suggest joining the AC custom shaders Discord channel, because the most groundbreaking stuff is coming from there right now.
Anyone who follows all of my above suggestions would have access to the vast majority of quality mods for AC, without having to beg and pester people on forums.
But on your point about the wildly varying quality of car mods: I totally agree with you, and it can be very frustrating having to bugger about with the crap quality cars. I find most of the good quality car mods to eventually float to the top though, and people like to share quality cars around, so that's where forums like this one can come up with the goods, because people regularly share good quality stuff they've found elsewhere. But this is not the place for people to be asking for specific mods, the place for that would be RD Forums or anywhere else you can speak directly to the modders themselves (Facebook groups are prob the best for this). But while Facebook mod groups are probably the best resource for car mods, just be prepared to sift through crap ones to find the good ones.