Need Help!!!

I'm a new member to the forum but have been driving Tacomas for over 10 years. So hello to everyone!

I have a sh*tty situation on hand and need some answers and opinions asap!

I just traded in my 01 v6 Prerunner with 230k for an 08 4 cyl Prerunner with 100k. Everything seemed good with the new truck until a week later the check engine light came on!
It's reading P0420 and I now hear an annoying rattling under the truck when in low gear which I assume and was told by someone that looked at it briefly is debri or broken parts in the catalytic converter clanking around?

I called the used car dealership and even though I bought it "As is" they were willing to have a local place throw in an aftermarket cat for $400 and pay for it which is cool BUT here is the bad news, this Tacoma has a 2.7L AEM cat NOT AEH which I've been told by multiple muffler shops can only be bought from Toyota and it's expensive as hell!!!!!
Quotes I got were $1750-1900 from mechanics installed and $2100 installed from Toyota dealership.

I called the used dealership I bought the truck from and told the guy this and that the $400 installed cheap cat can't be legally installed on this truck in California and now I haven't heard back from him in over a day and am afraid I'm gonna get stuck paying the full cost since they have no legal obligations to pay for it. The truck did pass Smog and I have a year registration from the dealer paid for but now I'm worried that they ripped me off or turned off the code to pass the CA emissions.

Is there anyway around fixing this or does the AEM Cat just cost a ton of money??? Also I'm worried about other issues with the truck that would have caused the cat to go bad. I replaced the cat on my old Taco but it was only $450 cause it was AEH and it didn't go bad until 200k miles, the truck I just bought only has 100k miles on it. I appreciate any advice and I guess I should have done more research on the vehicle but I'm stuck with it now.