The Music Man was the first musical I just had to have the album for....and it was the soundtrack to the movie, not the original cast recording...
and then in high school everyone in my youth group at church just had to get Jesus Christ Superstar when we saw the touring show in San Antonio
As a little me, I wasn't in the know about original cast recordings -- musical neophyte that I was back then, I just wanted an album with the music so I could listen to it over and over...
So I would have to say that The Brain really is the first show I saw where I eagerly awaited the original, one and only, cast recording...