No need for a humidifier, if you need anything it would be a dehumidifier.
What you need is something simple like this for temp/humidity monitoring:
http://www.walmart.com/ip/16888914
Keep your humidity between 30 and 50 and you should be fine. Keep your temp between 60 and 85 and you should be fine.
Most importantly don't get busted due to smell, you need exhaust with a filter or at least an air purifier with a carbon filter in it.
3x5 is pretty small for a decent sized garage so you may be able to handle it without a normal exhaust.