The wheel is a good starting point, made from steel or aluminium and painted more often than not. This is not going to last if left unprotected, any paint will fade and could even peel  causing you the job of repainting or worse.

Inside the wheel you have the brake drum or disc, normally made from cast iron and not very good at repelling the elements. Rusty brakes means a strip down and or replacement of parts, this will be expensive and could have been avoided.

Inside the brake hub or disc you have wheel bearings, this is normally well greased and has a seal that is designed to keep water out which is great when the car was new but your old classic is far from new. Water will find its way into the bearings and again will cause damage. Replacement and repairs will be the only way forward at this point and again your hand will be reaching for your wallet!