hey there! i recently bought a 2006 GX470 w/150K miles on it... here are my thoughts on first gen (ie. 120-series 2003-2009):
- afaik, no factory lockers available on GX (first or second gen)
- first gen vvti models have been known to suffer the AIP/valve stuck open issue. not sure if it's real or just internet hype, but AIP bypass kits are available and a good preventative measure.
- gotta watch out for rust... and the GX suffered the same fate as a lot of toyota/lexus models of that gen - cracked dash!
- rear air bags... comfy, but definitely has a shelf life, so either need to replace airbags eventually, or can just go with coil kits
- unicorn GX's are the kdss equipped NON-NAV versions... non-nav allows you to easily upgrade to a modern double DIN headunit, instead of medieval technoolgy from mid-00's... the non-nav conversion is a royal PIA
- i'm sure there's way more... but that's just top of my head stuff

honestly, i was going to buy my brothers 2004 t4r se v8, but SURPRISE SURPRISE he wants to keep it (at least for the time being...). he is the original owner, and i was waiting for a while for him to sell, and it just wasn't happening. anyhow, just to be different, i decided to go with a GX. still love his t4r, but i really like my GX... luckily i found one with a nice clean undercarriage (i.e., no rust) but it must have sat in the sun forever. all the rubber was dead, so i replaced the door belt mouldings, front inner door panels, wipers, and like i said the dash is cracked. had to also replace the radiator, but the PO replaced the timing belt and water pump just before selling (common maint). otherwise, just been collecting fun parts (like suspension refresh/upgrade, tires, etc.).

my wife actually has a 2015 GX460, and my 470 feels a lot more peppy... but obviously hers is nice and fresh, and mine is a little crusty!

i'm not sure if this is frowned upon, but i got a lot of info from ih8mud gx forum:
120-Series Toyotas | IH8MUD Forum