are you doing a wall between the stall/isle & the arena? or just a lower split rail fence type of thing for the arena?
I could see arguments for either option:
Wall- while working in arena won't be distracted by horse in the stalls
No Wall- better air-flow in the building, easier for you from the front of the barn to have an idea if something is going wrong in the stall in the back corner, easier to keep track of employees.
if you are going with a wall I would suggest installing a tube-ventilation system over each row of stalls (may still be good idea without wall) but it will bring fresh air into each individual stall 365 days/year without creating a draft.
you also mentioned horses staying out 24/7, if that was the case I would probably build the barn as you are planning but only put in 1/2 the stalls then maybe set the other 1/2 of the barn up as run-in's or put the stalls in later as you see more $$ and a need. thinking if you are bringing horses in the barn to eat you can bring in the first 11, feed them take them back out then while you are putting one horse back out bring one horse back in and use the same 11 stalls over again if it's just feeding time. in the really cold/wet days when you do want the horses in if they are used to being out together you could set up some temporary panels in the arena or the unused side of the barn since if you are looking at leaving them out most of the year anyway you may need them in 30-50 nights/year (fun winter it has been) but leave a group that is used to being pastured together to have the arena overnight... pain to clean? yes, cheaper than 11 more stalls for those occasional nights... yes
just some thoughts, but my #1 would be ventilation since it seems a lot of horse barns are built to make the people feel good but not necessarily the horse