He says he wants to run standard boost and so guess engine would be standard, you would need a chip to suit to start with as a different turbo, but none of the shelf chips cater for a T38 on an otherwise standard car. Personally I would stick with the T3.

If you require a T38 then you need the other components to suit such as injectors, MAP sensor etc.

So once you decide which route you are going to take and get all the hardware and a chip to suit of course IMO it makes sense to swap it all at the same time.

As said T34 is the most common for stage 3, around 380HP if setup right which is more than enough really.

Martin