Shabam wrote:
Plastic won't rust.

So I guess that's good?

Payne is cheap builder grade Carrier

You mean it's low quality? What would be a better brand?

My air conditioner is about 13 years old. The guy suggested I should
replace the inside unit and the outside condensor (3 ton) for $3,000
total. It will be Payne. Is this reasonable?

Once again, if the coils are not leaking just repair the pan. Plastic
is fine. Looks like you are in southern California...costs are higher
there but about $300 should do it.