The ARIA Player is only available as part of Finale, or with a Garritan library like GPO5. So, buy purchasing Finale, you already have one Garritan library.

Yes, you can make audio content that you may sell. All the Garritan libraries' EULAs say:

You can use these sounds in music productions, public performances, and other reasonable musical purposes within musical compositions. You can use these sounds in your own musical compositions as much as you like without any need to pay Garritan Corporation or obtain further permission. 

They also ask that you credit Garritan in any written materials, where possible. 

https://www.makemusic.com/terms-conditions/

What if I downloaded it from https://makemusic.zendesk.com/hc/en-us/articles/360037425313-ARIA-Player-Version-1-959-Installer ?

Then you'll have the Player, but no instruments to play on it. There are other compatible SFZ-format instruments that you can download elsewhere, of course.

Oh I'm sorry then that's the question I should have really asked from the beginning, if I'm legally allowed to compose music with Aria and free instruments and sell it. I think you implied a yes.

To be certain, you would have to check the Licensing Terms of whatever free libraries you used. Though it's unlikely that free audio products would restrict you in that way. 

Best of luck with the business. Making any money from music from music is difficult at the best of times!