What do you mean when you say "i'm not sure its necessary to be stainless (that close to the turbo)"
I wouldn't use anything else. If I had some mild tubing laying around and felt like it - I may use that - but stainless is where its at for exhaust IMO. I've found it can distort alot while welding though.
But yeah, ATP turbo has *EVERYTHING* you could want for building that. Minus perhaps an engine flange. IIRC, they have a T3 to 3" vband adapter they sell:
Here's one:
http://www.atpturbo.com/Merchant2/me..._Code=ATP-FLS1
Another:
http://www.atpturbo.com/Merchant2/me..._Code=ATP-FLS1