To handle the P0300 code, I will replace all 6 spark plugs and all 6 Coil packs. (something I had planned on as I just bought this car 3 days ago.) That should take care of that problem if that is even the cause of it.
Since I have it broken down that far, I'm going to go ahead and replace the PCV valve since the car has 174,000 miles on it and I don't know the history. I'm already going to be working in the area so might as well knock that out.
Make sure you check the new valve before you install it. I just purchased one that allowed me to blow air in both directions. This is not ideal. As a result I still have the old one in that I check every oil change.
P0431 could be a number of issues. more than likely with this year model, it's going to be the cat but I will check the O2 sensors first because I don't have any "strange" smells happening.
At this point I would not imagine you would smell any sulfur smells and the catalyst is likely destroyed (assumption here). You could crawl under the car and shake the flex pipe to hear if there is anything loose inside.
This is what mine looked like when I took it off
If the catalyst has started to disintegrate people have reported a possibility of pulling this material back in through the EGR
Whats your oil consumption like? granted its new to you so you likely don't know the full story there.
Question. How likely is it that I destroy my engine by driving with bad cats for a while? I don't think I will have the funds to fix it right away after the purchase of TB and coils but I need the vehicle sooner rather than later waiting on a catalytic converter fix. I know that if I have to replace it, I might as well replace the whole system all at once. It would be nice to have an exhaust system with a deeper sound to it anyway.
It is always a gamble. When I bought mine it had been driving like that for at least 2 years. I currently consume about 0.5qt of oil per 3000mi so I think I may have dodged that bullet.
In the end you have to do what you must and there can be good knowledge gained by driving and testing before you sink a ton of time and money.
Your plan seems reasonable to me given your resources.
Come back for help if you need it. Many of us have walked this road (myself recently) and would love to offer assistance where we can.