I could be mistaken, but you can probably find an original Island set for around $25, and the likeliness of it being in great condition are good. That's the kind of set that the first CD or two get played a couple times, and the rest never get touched. I didn't check Discogs before typing that, so I hope I'm not wrong.
I would encourage you to give organ drone a chance. It's actually one of the more vibrant --and dare say, dynamic-- types of minimalism. There can be some stunning vibrationis and harmonics. It can be truly beautiful and thoroughly engaging, while every bit as meditative.
If the harmonium is an instrument that interests you, I would recommend Charlemagne Palestine and Keiji Haino over Nitsch, particularly Palestine. In truth, Palestine is the more skilled organist as well, but Nitsch remains no slouch. I'm a big fan of Nitsch, but that appreciation is a little less musically pure and more forgiving than my fandom for Palestine. Palestine is a clowny humorist, and that is cause [for me] to isolate the sounds from the artist. My feeling is that Nitsch is more a theorist than a composer, while Palestine is a musician. Palestine understands tone and structure at a greater level. It's like apples and oranges, but while I'm fumbling around comparing the two, there you go.
Charlemagne Palestine - Schlongo!!!daLuvdrone (or any of his single-track organ releases)
Hermann Nitsch - Komposition fur Orgel CD, Orgelkonzert das zucher konzert 2LP
If the Palestine happens to grab you, give the JLIAT - 16:05:94 CD a try. Or even if not, give it a try anyway. It's a master drone work.