i have done it twice. both in the fall, low water. both with motors which were heavily used, though not all the time, from below westy to the rapids in cat and then below them to the lake. tons of super slow water in there. moab to potash is more less a lake.
i think my first trip was 18 days and my second 16. both involved layovers in cat for hiking.
plan for wind. last time we were motoring through the moab daily and could barely get downstream with the huge winds. commercials in the daily were pulling over and taking out above their planned takeouts and one group lashed on and got a ride from us. not sure how common huge winds are in that area. sustained wind could seriously delay a trip of that distance without a motor. if you were on a hardened schedule it could mean a true sufferfest. (this is where randaddy chimes in and says what a real man would do, we all know how he feels. save us the lecture.)
if you did it at high water (which i'd like to do sometime) you could forgo the motor and make it in a reasonable amount of time without rowing hard, day in and day out should you get sustained wind.
not sure about lake levels right now. it was up quite a bit but they have been dumping a bunch of water into the grand all winter.
as mentioned, westwater will be your only permit obstacle, secure that and youre golden.
dolls house is sweet. we rimmed out in X( or was it Y???) last time and it was pretty cool. lots of good other short hikes to ruins, pertified wood and all kinds of stuff down there. most of it is in the belknap. hike around and find something new. one of my favorite trips on years when permits and invites come up short and im looking for a long getaway.
if you can get people to come in and out you can make the shuttle easier and cheaper to deal with, as well as get fresh supplies.
hope that helps.