We picked up our rental car at Dollar at about 10:15 a couple of weeks ago. All the desks were open -- no lines!
I wouldn't travel to arrive so late again, but that part of the journey had no problems.
The only thing you might want to do is find out what happens if your flight is a couple of hours late, in case they close at midnight or 1, but I'm sure there are contingency plans.
The airport was quite lively at 10-10:30 on a Weds night, and the roads between the airport and WDW are very brightly lit. There's construction going on quite a bit of the way overnight, but there's so little traffic it probably won't affect you much, except to slow down in their work areas.