Marty:
The "spring air" guns require special oil. Using the wrong il can cause the oil/air mixture to combust, called "dieseling". This can cause damage to your piston and other internals. It sounds like a .22LR when this happens.

SG:
With CO2 guns, you have a drop off in speed over time, plus the added cost of CO2 over time. I do like them for the reason of the fixed barrel and the quick reloads, but the cost of CO2, and having to carry them, is a big downside. Imagine a tornado of crows above you, with more in the trees, and you run out of CO2. Buzz kill.

Accuracy wise, a good break action spring air RWS is every bit as accurate for you and me, as a CO2 gun would be. RWS does make a few side levers that have fixed barrels, but I'm not sure of the cost. My .22 cal Webley shoots dime sized groups at 40 yards. Certainly accurate enough for head shots on starlings and crows at that range.

Scope wise, you need to use a scope specifically designed for spring air guns. Spring air guns have a rather violent recoil impulse and they will trash your average .22 scope, or even cheap Tasco full sized rifle scopes. The cross hairs on the spring air scopes are build sturdier to take the additional recoil. Make sure your scope ring bases either have an integral scope stop, or buy one. About $15, and it keeps your scope from moving, again due to the recoil impulse.

I use iron sights on my .177 rifle, but on my .22cal rifle I have a Beeman 4x scope with a parallax compensator. It is 12+ years old and keeps on going strong after 3,000+ shots. Irons vs. a scope is a personal choice. At 40 yards and under, irons should be fine, but a scope is nice.