Can you get more pictures of the downpipe? Like of the part where it bolts up to the turbo?
It all depends what you are after. If that downpipe will be a good enough upgrade over the stocker, that is good. If that catback is 3", you'll have a full 3" turboback for $300, which is very cheap. It'll probably be loud, may fit weird (may not) and some welds may crack (again, may not), but you'll have a $300 3" turboback -- not many Subaru owners can say that.
Alternatively, you could get a used known-brand catted downpipe for $300 and bolt it up to the rest of your stock exhaust. You'll still make Stage 2 power, and it will be quieter. You may not make as much peak power as with the 3" exhaust, but you'll still be at Stage 2 and it will be very quiet.
Both of the choices above will need tuning.
If you decide just to use the catback (midpipe and muffler), if they are 3", you'll need an adapter to hook it up to your stock 2.5" downpipe.
Your choice, either choice is fine. Only way you can go wrong either way is if parts are bad -- then you are out $300.
Stan