I did the same thing about 8 years ago. My room was a 20 by 14 framed in deck. I insulated and finished it like a normal room. I have 3 windows along the side that I can open for a breeze and give great ambient light. I have a over sized door going directly outside too. Allows for heavy objects to come through the door better, like a gun safe. Along one wall, I built a bench. I use this bench for reloading. I store my hunting clothes, reloading stuff, gun safe, etc in this room. It is also my workout room with weight bench and weights.
My advice is this, you can never have too many benches. This summer I'm going to build two more. Trying to single stage and progressive load off just the one, plus use it for fletching and bow work, is getting cluttered. For benches, make them super sturdy. Frame them out of 2 by 4 studs, and plate them with 2 by 6. It allows for presses and such to be screwed directly to them and you don't have to Worry about ripping them out of the table after a couple thousand pushes on the handles.
Another option for the top of a reloading bench is the metal clad wood from inside treadmills. If you have a fitness equipment store nearby that sells and repairs treadmills they probably have a few of these kicking around. A guy can usually get them for free, they're a great size for a bench top (mine is 60 X 32) and they're incredibly strong for mounting a press on.
