Someone I know is selling a H9, and they've maxed it out using the max out program. Am I right in thinking that all the algorithms come with it so it'll always be a 'max'?

Yes, once an H9 is maxed out, it is always a Max even if it's sold to someone else and registered to a different account.

What happens if new algorithms get added? Will I be able to download those too?

Yes, you use the Restore Purchases function in the Settings section of the app to install newly released algorithm to your H9 if you're a Max owner.