Co2 bottles have a 5 year test. I believe the reason that co2 shouldn't be put in an air bottle is that it is a 'wet' gas, creating moisture whereas air is not.
Why do you need a bottle and antisyphon but not a valve?
Powerball (and other paintball suppliers) can supply co2 bottles with antisyphon valve attached pretty cheaply. Most places won't supply the bottle without the valve attached nowadays since earlier this year a couple of valves and bottles parted company whilst gassed up and sadly a couple of bystanders in the States were killed as a result of poorly fittted co2 valves. Yes you can buy second hand bottles on p8 but unless you KNOW how to fit a co2 valve correctly (correct o'ring size /torque/loctite etc) don't even consider saving of a couple of quid over your, and everyone elses' safety!