Ken A......plugs will last a long time but best to remove and check them during annual services. If they look ok and gap is ok then just refit them. Remember...DO NOT overtighten them!
I do 7k+ miles a year and annual checks at service. I put new in 2 years ago so I had a starting point for life for my own satisfaction.
A warm engine will evaporate any water in the plug wells but a cover will prevent rain getting in on a cold engine.
To me your rusty plugs suggest previous owner may have stored the car cold in wet conditions with little mileage done.
Mine is garaged and I use a full storm cover when out and about that covers bonnet louvres. John Taylor Phoenix make. Excellent cover.
