The 182 manifold will require you to get an exhaust system that is matched to the manifold, as this is different on the 182 to the 172. I would say that the 182 manifold would be the best bet, unless you can get these custom ones on the cheap for us all
If you are getting a de-cat, youre gonna have to get something chipped to make it of any use, mine seems to have buggered mine up a bit at the top of the rev range... Flat spot from about 5.5k to about 6.6k