Welcome to the Bunnings Workshop community @kelsroo. It's amazing that you've joined us, and many thanks for your question.
I would suggest you start by testing the painted surfaces to see if they are sound enough to coat with paint. The paint looks quite degraded from the images, and this is not a suitable surface to paint directly over. Run your fingers over the paint and check to see if it leaves powdery paint on them. If it does, this is a good sign further prep work is required. You'll need to sand the whole barrel down until you reach paint that is not powdery, or you might be able to remove the powdery paint with soapy water and a scourer.
It would be worth pointing out that the rust on the inside of the barrels will eventually eat through the steel and flake your paint off. It's a bit hard to tell how long this process would take. You could paint the inside with the rust converter, extending their life by a few more years.
Please let me know if you need further assistance or have questions.