You DO NOT want O2, you want HPA (high pressure air) bottles, and you DO NOT fill CO2 bottles with HPA. With some of the guns mentioned, you would only need a quality floor pump (Hill, FX, Benjamin) to fill as long as fill pressure is around 200 bar (3100psi) or below. If it's above that, lots of pumping and the pumping gets much harder, and not all floor pumps are rated above 3300psi.
Re: the squirrel problem. Handling it responsibly and legally yourself is much cheaper than constantly hiring professional critter control, and consider $500-600 well spent as "preventative" as squirrel damage can easily exceed $1000+. Just check your local ordnances, laws, hunting laws, and check for exceptions that allow you to protect your property from damage by pests.