CO2 is the easiest to use, no pumping, no cocking a big spring, just pull back the bolt, load, and shoot. And no recoil. But you have to buy the CO2 and it doesn't work too well at cooler temps. Crosman CO2 guns are popular especially with folks that like to customize their guns. They are easy to work on and can be quite accurate. I have a couple CO2 BB pistols that are fun for can destroying.
Springers are the next easiest to use, but depending on the gun can be rather difficult to cock and shoot. And they can bite you too if you're not careful. Something in the 500 to 800 fps second range is good place to start. Springers have a weird backward recoil plus a normal rearward recoil that makes them more difficult to shoot accurately and it also is mean on scopes and mounts. Some folks that jump right in with a 1000fps or greater gun and have never shot a springer get fed up pretty quick, especially if they have a lot experience with firearms. Learning the proper methods of shooting a springer is easier, at least for me, with less than 1000fps. The springers I have would be considered junk by a lot of airgunners. Cheap Chinese guns that cost less that $50 each that you have to tweak and modify to get them to shoot right. But I like to do that kind of stuff. A lot of folks don't. A lot of folks prefer that their springers are made in Germany. I have a really sweet shooting old Russian break barrel I picked up for cheap at a gun show.
Single pump guns are pneumatics, but as the name says only take one pump. Also no recoil. The Daisy 953 is a single pump. I'd like to get one myself some day. I have read they are a bit hard for little ones to pump, but they are almost a 10 meter competition gun in accuracy.
Then there are the multi-pump pneumatics. These guns generally have low to no recoil. That Crosman American Classic pistol you mentioned is now known as a 1377. It is one of the easiest guns to modify. You can order a shoulder stock from Crosman to convert it to a carbine. They also have a steel receiver kit that is grooved for scope mounts. The Remington Airmaster 77 has a loyal following too. The sweetest shooting non-modified multi-pump I have is an old Crosman 66 Powermaster I found at a thrift store. Dime-sized groups at 20 yards. Then there is the Benjamin-Sheridan line of pumpers from Crosman. Classic good looks and all metal and wood.
If all you are shooting is paper and cans, .177 is fine. If you want to hunt .22 is better. I don't hunt, but still have a couple of .22's. A Crosman 1377 pistol that has been re-barreled and a Crosman 2200 pumper (no longer made). The heavier pellets make a satisfying thunk when they hit! .22 pellets are generally more expensive and harder to find locally than .177.
Most any of the guns under $200 will need some kind of trigger work. Some respond well to a little polishing, some almost require aftermarket parts to work well. The Gamo triggers are notorious for this. Most of Crosman's springers are Chinese made copies of Gamos, right down to their triggers.
Hope this helps.
