not sure on the rental situation, but i'd imagine there would be options in Moab...potentially even renting from the commercial guys (as September is likely a pretty slow time of year for them). I too second Coyote shuttle, they may even be able to get you in touch with the right people for the rental stuff.
Anyway, expect super super low water (~2000 cfs pre confluence), which in turn will mean a bit float down to the confluence without a motor assist (3 days). However, September rain in the Rockies can happen and spike the river quite dramatically sometimes, i just wouldn't plan on it. Lower Brown Betty is my favorite Camp in Cat and has great access to the Doll House, so a layover day there is my recommendation. Anyway, should be two solid days of white water from there before reaching what was Lake Powell when full. I won't be surprised at all if you actually have current all the way to the take out, as the lake is over 100 below full and likely will only be lower by then. The problem however, is that there are no longer good places to camp because of the silt deposits that built up when Powell was full. So....i'd try to camp near one of the last rapids and make a big push out the next morning before the infamous afternoon winds pick up.
If you are on a tight schedule a motor would be nice, but it's also going to be one more piece of the puzzle you'll have to put together. If the lake were high, i'd say it would be a must but it'll basically be a river from start to finish, just a slow one for most of it. Without a motor i'd recommend 7-8 days with a layover day in the middle. just take your time and enjoy the scenery you won't see too many people down there that time of year. With a motor you can easily take 3 days off that.