what I would do is find out if the downpipe flange is the standard 5 bolt t3 (and I will probably look that up now that I'm curious) and go from there. This way, you can have a turbo that will not max out your engine (with tuning of course) and when you're ready to go bigger with it, find a larger turbo with a t3 turbine side that is more suited to the engine. That will be the easy part, swapping to a larger one.
edit: looks like it is in fact the standard t3 5 bolt flange. All I can say is just make sure you don't try to push more than the turbo is good for or you'll be asking for his other one. They say not to go past 16 psi on OUR (mazda) engines, however the focus I don't know. I wouldn't push it more than that.