60 x 120 is a perfect size riding arena for a big horse and the minis would require much less. My arena is that size and when I was driving used a lot less than that. When I set up my ranch for the biggies I put a lot of thought and looked at a lot of other ranches before I set it up. I ordered bars as stall dividers for the tops so they weren't confined by solid walls and helped with the air flow during the summer.
Now that I have minis I have given much thought into what I would do differently. When I have horses in the barn they are in solitary confinement because they can't see over the stalls. My idea now and I will put in the terms of the barn you are building, would be to use an end and not put stalls in the middle. I would find it inconvenient to always be walking through the dirt to feed, clean, dragging hoses to water. I am always thinking about new configurations to make it easier. By having stalls near a door would cut down on a lot of needless footwork.
I don't know how you store your hay but having an area on the corner of the arena by the stalls would make it easier to put hay in the barn and it would be right next to the horses.
I have seen, and loved the idea, of buying premade chainlink panels for the stalls that you just bolt together for whatever size stalls you need(bigger for foaling, smaller for stabling). They can see each other, it gives good air ventilation and the best part is, when you don't have horses in the barn you can easily take them down and use the whole arena.
As mentioned before, most of your driving work is turns, serpentines ect. a "track" would only allow mostly straight work. By having stalls on an end, you could put jumps or a trail course in the middle and still allow enough space for turn work. You could use the inhand obstacles to drive through and around giving another dimension to your driving practice.
I don't know how many horses you plan to stall and how often they would be installs but mine love being out in the rain, mud, snow and if I don't have to always be mucking stalls then I am much happier too.
Hope this gave you something to think about.