csmooth

So Im new to the forums and new the whole Audi life so Ill use this as my beginning post an fill you in a little before I go into further detail.
Bought an 04 A4 off my stepdad a few weeks ago, 1.8t Quattro 6 speed manual, and all in all Im rather impressed with it so far. Never driven an AWD car and I must say its a great feeling.
So it has some minor fixes Im working on interior/exterior wise (i.e. glovebox/center console broken *ordered*, tail lights have spider cracking, hood pop and DR door handle dont open well, and a grinding scraping my tint in my passenger window) but Im planning on doing work performance wise down the road. Yes Ive read all the posts about it, no Im not jumping into anything. I understand the concept its smarter to chip first and APR suggests fuel upgrade and exhaust after chip.
Ive been a certified welder for going on 8 years, and I think while I save for the chip upgrade, I think I want to make my own turbo back exhaust.
So.... Im looking for any insight on things I should keep in mind. Im seeing 2.5" from turbo into 3" is pretty common and good for low end but limits power on the big end, where straight 3" is the exact opposite. Im not new to doing this Ive actually done a few for cars Ive owned before (Genesis 2.0T, Tiburon GT). All I know right now is im in the air over 16g T304 or T409. I mean its going to be under my car and I live in PA, so snow/salt etc, I would rather do T304 unless anyone can tell me other wise. Im also curious as to what all components people are using and like, i.e mufflers and cats (i live in a visual emission county). Any suggestions on what sounds good/preforms well? Just feel it makes it more personal to do it myself and adds to the built it my self kinda thing.