I would never hire someone over the net who I paid hourly, because there's no way to verify the number of hours they actually worked, and no way to guess at what your final cost might be. You could of course fire them if they didn't work fast enough, but giving someone one week at $150 an hour to prove their worth could end up being a very costly mistake.
If someone wants to be paid by the hour, I usually ask them to work out how many hours they expect it to take, add on contingency hours, and then give me that number of hours times the hourly rate as a flat fee.
There are valid reasons for not being able to do it, of couse, but I feel there are valid reasons for being concerned if they don't. If they can't do it, I'd be worried about their time management abilities. If they won't do it, I'd be worried about their honesty. My bottom line is that if people are forced to quote this way, they never underestimate the number of hours it will take in order to make them look more attractive than anyone else interested in the job, so I don't have anything to worry about.
I can honestly say that, of all the people I've worked with in any shape or form, I can think of only two I would give an open-ended hourly rate to, and neither of them has ever asked for it. Of course, it means I have a much narrower selection of people to work with because a lot of people ( quite justifiably, I admit ) don't like to work this way. But I only work on a flat fee basis too, so I can't be accused of hypocrisy.
Of course you can't really do this right now because you have a fairly open ended job. If you had the design doc much tighter, you might well be able to do that though. Anyway, just some ramblings in case any of it was useful in your thinking.