I figured that's kinda implied in the 'rare' part, but yeah agree. Always support as much as you can.

I figure masterings are entirely fair game if you own a copy of the album in another form. Eg I've got a bunch of MFSL, Audio Fidelity, AM+ and SHM-CDs that have great mastering of particular albums that I'd never have been able to afford as physical purchases due to their rarity and 'audiophile' collector status IRL. I have no doubt that the musicians and engineers involved would be glad their work is being enjoyed, rather than unobtainable.

Rare CDs that are in-print nowadays generally don't have any tracks (or masterings) that you can't get via a separate release or digital purchase for a more reasonable price, because modern artists mostly live or die by digital stream popularity.