Scratching head P0442 P0328 P0456

I have 2004 Xterra with engine light error and originally had some bucking after returning from 2wk trip and sat during over 7 days of rain. I had my car at a mechanic who told me I need a new head gasket. Never ran codes. Took it to another mechanic who said there is a leak but with she and 165k miles to trade in.

So I ran the check engine light codes that neither did as I got pretty worried and I got the following errors
P0442
P0328
P0456

When I read about the knock sensor, I can now say in hind sight some issues like lower fuel economy and occasional smell of fuel. Neither symptom waa consistent. I have had to tighten fuel cap before too and engine went off.

So QUESTION: I am reading all these, and how do I know if I spend the money to replace knock sensor (btw neither mechanic mentioned this), spark plugs, and maybe gas cap that this will fix my problem or do I have a major engine issue?

I would love to keep my truck especially after seeing how crazy the used car market prices are, but felt like I had one mechanic instill fear.

Need some guidance. Thank you in advance.