Does the suit have an expiration date? I understand they can become ineffective after a certain age.

Tyvek is a polyethylene based material. It provides small particle protection, but it is not impervious to liquids or gas. Over a period of years I cannot see why this material would be ineffective for dust or particle protection. I've not see any expiration date on any of my suits. The weakest point is probably the small area of cloth that binds the zipper to the fabric. The Tyvek material is very resistant to tears.

How does a Medium fit? Especially for a 5'2" person?

My wife is 5'2", 104 lb and the medium suit is a bit long for her but the torso is comfortable without being too baggy. The legs have an elastic gathering at the bottom so the longer length is not an issue.

Are these brand new and individually packed?

They were brand new and each came in its own plastic bag.

With appropriate facemask, gloves & boots are these satisfactory protection from radioactive fallout? Hold up in rain too? Better option?

Holds up in rain good. I wouldn't think it would protect you from radioactive fallout though -- hopefully Trump will get impeached before we need to find out.