First, forget the 75s, if you want to hover at all they just won't do the trick. OS91 is really your best bet here.
I've owend 8 of these (but not one failure that I didn't cause), everyone had an OS 108 for YS110. Over powered? yes, but if your learning to hover you will want to be able to poor on the coals and get #$LL out of there. Just don't do any full throttle down lines and you'll be OK. It's not the amount of power, it's the speed, which results in flutter, which results misery. FYI, I mounted them all horizontally, the cowl will look a little hacked up, but less trouble keeping them running and that's VERY important for 3D down low.
Hope this helps a little.