Hey
I have a Version 9 Sti. So here goes my attempt to answer your q's..
1 - Yes, mine has 2 o2 sensors. One right behind the turbo, and one at the other end like pictured.
2 - No the factory down pipe has 1 cat, and another small muffler (I think) in it.. Definatly 2 mufflers in it though, but one 1 is a cat i believe.
3 - Yes, looks to be in the exact right place
4 - Yep, if you get an exhaust for the right model, it should bolt straight up to the downpipe.
5 - Only thing i can see wrong with it straight away, is that it steps down to the factory size exhaust at the end, so if you get an aftermarket 3" exhaust, it will have to step down to factory size as well to bolt up.. But you could chop the flange off, and get a new 3" one welded on, bit of work, but not to hard to do.
Cheers