It's been 4 years since I put together an OL team. Back then, you got 2 AAA pitchers and could get by with a pretty low amount of originally drafted IP.

Without AAA, what are people seeing success with? How much does it change if your pitching staff has a high vs low K/9?

You were a high quality player the formula has not changed since then so you should have a good idea. But for everybody else, here's my opinion.
1375 or so innings is about right. 1350 if you don't mind a risk of slight fatigue. Or you can draft an extra mopup quality starter just in case and hope you never need him.
High K pitchers get a few more innings because they don't get all of their K;'s which lowers their pitch count, high BB pitchers lose for the opposite reason,. Good defense lowers IP by a modest amount.. using Waiver Wire wisely can save you 40 innings or so, just keep rotating your cheap low inning SP after seriously fatigued by a start.. Trades erase fatigue too, and that's why I veto trades for fatigued dudes and you should too. Ballpark selection matters too especially a low 1b rating.
I haven't played in an open league in forever so my answers are based on theme leagues.