Burning leaves is just not something you do around here. I'm surprised it's allowed where you are.
I don't think you want to be removing your vac unit every time you dump....actually I KNOW you don't want to be removing it...scratch that plan...really. I have a low profile dump trailer and love it...nice low loading height for lifting stuff in and if you want to ramp equipment in. I see that Bri-Mar has a ramp gate. That new one looks like a really good deal and looks like a Bri-Mar copy. I think the seller is overstating things a bit calling that Bri-Mar in excellent shape but it looks better than the Proline though. Both good tough trailers, but by the time you get done shopping them out you'll probably be at or above the price of the new one.
You can probably get by without the wheel blower, depending on what backpack blower you have.
Most people mount the vac on the tongue above the battery box, either attached to the frame or the box itself. I feel cramped working in that area between the truck and trailer but people do it all the time. I have a 16HP Billy Goat I mount on my trailer door. If you're going to mount it on the back of the trailer like you mentioned, I think you need a swing-away hitch or something.
Sorry if my thoughts are a bit random...no time to compose this any better...working on my invoices.