SEARCH BUTTON Is your freind!

There are a lot of threads about turbo's for GT's! Anyways
-DA Sport
-C2Gas
-SF
-Custom
There you go. If your planning on buying manifolds and everything peice by peice, you willn't save on costs and risk mixing hardware that isn't compatable. If your looking for those things I suggest going to a local shop and asking them about cusom manifolds andcustom piping. Pretty much a full custom turbo.
Just thought i'd help clear up some on your idea's too. Manifolds relace headers. So you willn't have headers. You will have a manifold.
As far as Internals.
www.monkeywrenchracing.com [
monkeywrenchracing.com] has your pistons and rods and vavles. Your stock valves don't have to be replaced though.
** Or you can wait because I might be selling my pistons and rods. Which ARE BRAND NEW, not even out of box.**
There are no turbo exhuasts out for the celica. Atleast no turbo backs. ou'd have to get it custom. or if you like your 2.5pipin stock just get TRD or TPR exhaust

Your turbine is part of the turbo so don't worry about it!
FMS options are The "Black Box" frm SF. (Not recommended) or PFC or GReddy E-Manage.
As far as hoses and BOV and intercoolers. Those are genaric.
Overveiw_
I'd recommend that if you are serious about turbo'in that you realize that you will be spending a lot of money, and DON'T take short cuts! Turbo'in is all about quality. I'd recommend C2Gas. Heard nothing but good things and are great guys to work with. As far as DA's turbo kit.. Well put it this way know one has really heard from David in awhile... and SF.. There is a waiting list and will take months to get your kit. Custom is anyways good if you do it and can do it (may spend alittle more) but good and you'll have more controll over your power.
SEARCH BUTTON IS YOUR FREIND! there is a lot more info ot there..Just use the search button!

Hope this helps!