Black Sabbath worth buying IMO:
Black Sabbath
Paranoid
Master of Reality
Black Sabbath 4
Sabbath Bloody Sabbath and Sabotage are good too, but these 4 are the best of the best.
And Ozzman Cometh is mostly solo Ozzy stuff. Did you get the one or two disc version? I bought that album years ago and it came with a bonus disc that has very good alternate versions of Behind The Wall Of Sleep and Fairies Wear Boots. Every time I see that album now it's only one disc. I don't know if they just got rid of the bonus songs or if they moved them on to one CD.
p.s. Mr Crowley is a dope ass song.