I'd just point the tractor down hill and drill away. Just pick a day when the ground is dry and take your time and don't get in a big hurry. I'd also have someone else around to give you a hand so you can stay on the tractor. Almost without fail, I get some wood or ?? caught on the cutting teeth of the auger and have to knock it off (after shutting it off) with a shovel to make it work right. Around here, I'd figure about $250 a day plus delivery for a compact tractor with auger.
Another option is an auger on a small excavator. I know Cat makes the auger for their mini excavators and it looks to be very handy. No matter what angle the machine sits, gravity plumbs the auger and you just have to stick it in the ground. If I was doing a lot of posts on a hill, I think I'd give it a try. I usually rent their 304.5 and it runs $285/day.