This has been covered before. I will summarize:
You CAN kill almost anything with a Saiga-12.
A Saiga 12 is not designed to kill something on 4 legs or with two legs and wings.
Much like hunting with an AK derivative, if you have one already go for it, buying one to hunt specifically not a great idea.
If you are set on doing it the longest barrels is obviously the best for hunting(I think that will be about 20"). I don't think any have interchangeable chokes, which is pretty important for a shotgun you want to hunt upland, hog and deer. I think you would almost have to get them installed.
It CAN work, but other things WIL work better for the price.