I can help out with the autocross setup and a little on the track side as well. I would recommend the Porterfield R4S as a dual purpose track/street pad but you would be better off having a dedicated set of track pads as was already suggested. Fluid I will leave alone as I am running stock fluid right now. I do really like ATE Super Blue from prior use on different vehicles but I have no clue if they make it in DOT 4, even though I imagine they do.

A harness is great but an aftermarket seat will not be legal for A stock autocrossing.

As for tires. It all depends on how much money you are willing to spend for speed. I have running Kuhmos on the front of my car and Hoosiers on the rear. Kuhmos are much more durable but don't offer a 19 inch size. My rear Hoosiers have held up really well and have just past 80 runs this past weekend. Hoosiers on the front of my car only lasted 20 autocross runs where as the Kuhmos are past 40 and still have plenty of life left in them.

Welcome, these cars are pretty fun to autocross and race even though most people don't get it.