A nursing letter should pick two or three requirements from the posting and point each at a fact already on the resume: unit, patient group, or registration.
Open with the role and unit. One short paragraph per requirement you can prove. Close without adding skills that the CV does not contain.
Registration and right-to-work belong only if true and current. Do not invent ward experience to fill the page.
Usually well under a page. Specific beats long.
Yes when the unit and requirements change. Keep the facts; change which facts you select.