We are getting a hand me down stroller which is about 5 years old, for our baby which is due in a few months. Should we change the mattress and lining of the bassinet piece of the stroller or just give it a good clean?

Does this pram meet Mandatory Australian Standards?
Product Safety Australia develops & monitors all safety standards in Australia.
Here is their information for prams/strollers, which includes the following:

Only buy a pram or stroller that meets the mandatory safety standard.

Do not place a baby under six months old in a stroller until they are capable of sitting in an upright position.

Avoid sleeping children in prams or strollers. Never have an unharnessed child asleep in a pram or stroller as they can move about and may be at risk of falling or entrapment. If they are asleep, maintain regular supervision.

https://www.productsafety.gov.au/products/babies-kids/kids-equipment/prams-strollers

Red Nose also has the following information.
Never leave your baby unattended in a pram or stroller. It is not a substitute for a cot.

https://rednose.org.au/article/prams-and-strollers

A pram or a stroller is not recognised as an alternative safe sleep environment, particularly if unsupervised.
Red Nose ∏ Safety (ACCC) also has information on bassinets
https://www.productsafety.gov.au/products/babies-kids/kids-furniture/bassinettes
https://rednose.org.au/article/bassinets