Here's some guidance for you:
72F degrees is the highest I've heard - but that's hot. I know we like ours at 60-65.
Regarding outdoor stuff: temperature isn't the only factor to consider. One thing you need to control heavily in a datacenter environment is humidity. Outside air is probably more humid than your servers would like. Thing is, a dehumidifier runs with a compressor and condenses the air, just like an air conditioner. The thing about recirculated air is that you basically create a heat cycle (think thermodynamics). If you've ever been in a datacenter, you'll notice they have hot and cold aisles. Basically, what happens is they send chilled air through a cabinet and then collect the same air at the back, send it through the ceiling and rechill it in the process. If you bring outside air in, you're adding a lot of complexity to the cycle.
All that being said, I am by no means a data center infrastrucute expert. So if someone else tells you something I said was wrong, there's a good chance it is.