Thinking of buying a house. I know, not in this economy, right? But is an older 1953 house with 100 amp electric service OK or is it outdated? All the major appliances are gas so there's no big draw. If 100 amp is there and is working would I have to update it as the new owner if I didn't need to? (Not quite sure how to phrase this.)Also as long as there are blank spaces in the service panel to add circuits can I use those or must I stop if everything already there adds up to 100 amps?

Totaling the breakers is meaningless. There is a spec called number of poles that you must not exceed on the box. This is essentially the number of wires connected to breakers. There are a couple of ways to add more circuits than spaces.

Loads that are concerns would be central AC, electric stove, electric hot water, electric tankless hot water, electric stove and a hot tub.

You need to be concerned with the number of items on at the same time.

To really determine if the service is adequate, a load calculation needs to be done.

Usually the seller has to fix things that are flagged as code violations unless alternative arangements are made.