Russ, first let me welcome you to our site!
It sounds like your catyictic converter may be damaged or worn by the description you give. Those things can get old after a time and get clogged up. Does your exhaust smell abnormal or like rotten eggs? On the very earliest cars with cats they had some trouble like that but I don't think the later ones did but there is always a chance that a previous owner had the cat replaced and who knows about the quality of replacement parts people used these days. It was a good idea to replace the air filter box. With todays engines having so many sensors that the computer uses to control them it was probably a good move. About the grime on the engine, that would be normal from the power steering pump. Clean off the engine and monitor it to see where and how quickly it gets dirty again. On a nine year old car I don't think it is abnormal to have dirt from that, some pumps leak more than others and any dust at all becomes dirt as it sticks to the power steering hydraulic fluid. Use an old rag and some cleaner like simple green or orange plus and just spray and clean. I don't like using the products like Gunk engine degreaser because often you end up with a car that won't start afterwards. Not from the product but from washing it off with water. You have to be real careful with that. At any rate, good luck and I hope to have been some help.
