We recently sourced catless headers for our friends sons 02 gs300 and and it's been a long time since I've messed with this.

What all do we need in addition after other exhaust components to have the vehicle running properly?
We were quoted 2k by a tuning shop and that seemed extremely steep considering we are buying all the parts and we paid that for the car.

 

I went to Midas for cats that I bought from Rock Auto. Around $250 for install.

 

You'll just want to put some o2 extenders in for the two down stream sensors to avoid the check engine light due to missing cats. It is a very easy car to install catted exhaust manifolds or cattless headers. Finds somewhere else to ask or take a crack at it yourselves. I've done the swap in 15-20 minutes. There are just the three flange bolts underneath and the manifold to head nuts. Comes out from the top. Super easy if there are no broken bolt or rust issues or seized o2 sensors. Time is added for all that.

 

I find that 02 sensor extenders, with a small chunk of cat in them work best, my 1998 Lexus gs300 was straight piped and ran badly till I installed them and than the check engine light went away and car ran great and sounded good as well.