Musicals don't have soundtracks, movies do. Shows have Cast Recordings. You should buy: Ragtime, Seussical, Birdie, 110, Grey Garens, the Spelling Bee and the original, not revivas l of Company, Chorus Line and Chicago. Why the originals? Elaine Stritch, Gwen Verdon, Chita Rivera, Jerry Orbach and the original cast of ACL.
You should also consider getting those obscure ones: The 1956 Candide with Barbara Cook, 70, Girls, 70 and the 1947 Allegro among them. Why? Glorious Broadway scores as they were heard originally.
NYC Visitor and Broadway Fan