Should a fireplace be cleaned if it hasn't been used much and also for a very long time?

For sure you need to have the fireplace and chimney inspected and cleaned. If you are going to be using the fireplace then the chimney and fireplace should be inspected every fall. In your case you do not know what was burned or how much it was used and if the chimney is clean and open. A blocked chimney resulted in a four person family dying by CO poisoning because a bird had blocked their chimney. This is nothing to fool with if you are going to use the fireplace then you must service it.