You can check the cat by removing the pipe directly behind it and looking into the end. If you see chunks of material the cat is bad. You can also check it by tapping it with a rubber hammer. If you hear rattling inside the cat is bad. Don't waste your money on a shop.
The lifters are not adjustable so don't even bother. You should be able to quiet them down if you go with a little heavier weight oil like a 10w40 the Restore may help too. If you put a cleaner in the engine don't leave it too long as it can do more damage than good if it thins the oil or blocks the oil filter.
Keep me posted and I will help you all I can.