I think fast is relative. IMHO, and this is backed up by other Dory people, half assing a boat is only gonna save you a minimal amount of time and you'll regret no just doing it right with the right stuff. If you are just gonna toss something together, go buy a cheap beat up old boat.
Everyone I talk to says, at least your first boat, will be 300+ hours of work. Over 75% of that will be after it looks like a boat but needs all the fiddley stuff like hatch rims and such. This also assumes you have the space and tools and willingness to buy more. Definitely time for the OP to build one, but needs to start now. I know for many, like me, its just not in the cards to complete one so buying is gonna be the way to go.
The outfittes who have Dories don't really rent boats and they make their guides jump through a bunch of hoops to even get some time rowing one... no way they are gonna rent one out to a private boater.
So that basically leaves buying one.
Honestly, while I agree that there are differences between rafts and dories in how they do down the river... as long as you are proficient with a raft and can make it go where you want to you'll be fine in a Dory. You need to be much more vigilant and always be on the lookout for obstacles, but in most ways a Dory is pretty similar to a raft and you'll figure it out pretty quick.
I still haven't had a perfect run down the Grand in a Dory...always hit something down there. Luckily I have a pretty bomproof aluminum Dory that has seen it all before. I highly recommend one if you are sorta prone to hitting stuff. It shrugs off the little stuff and tends to dent instead of break for the medium to large hits.Some you can pound out, others you just deal with. Pretty sure I've take a few hits that would have been a really bad day in a wood or foam core boat.
When my Dory journey started.... I just asked on the Whitewater Dory page and ended up finding a great deal on my Dory. I can definitely sell it for what I bought it, but not sure that will ever happen. Good ole' Wesley will be sticking by my side for a while I think...even if it eventually gets a little brother.