Really, out of the above, I’d recommend the 19 track version + “Get Real”. It’s probably the best bargain, too.
The version with a second disc of songs like “Slip Away”, etc? That extra disc is a wholly unrelated Bowie album called Heathen, which is a fantastic record in it’s own right, but ultimately has nothing to do with Outside.
And the one with bonus remixes? I highly doubt that any of those are essential, and the extra track on that one, “Nothing To Be Desired”, is, urm… disappointing. As in, we almost played it on the episode since it’s a rare track, but at the end of the day, it’s just a really repetitive song that adds nothing overall and becomes a bit annoying. Your mileage may vary, of course.