Hi Matt,
Unless you have access to milling machinery, or know someone that can do that for you, modifying a pistol can be a bit of a challenge with just hand tools. It can be done, but few people have the skill and/or equipment to do it.
Beeman, Berretta, Crosman, Sig Sauer, Umarex and more pistols are available in CO2, If you are going to be doing pest control or small game hunting at short range with a Co2 pellet rifle look for velocities of over 700 FPS in .177 cal and over 600 FPS in a .22 cal. But remember that Co2 is very temperature sensitive. For best results shoot your Co2 pistols when the temperature is 70 degrees Fahrenheit or higher. So these pistols have limitations on both range, power, and temperature fluctuations in power.
Further, barrel length is critical in producing maximum fps in any type air gun. Depending on outside temperature you can gain roughly 15 fps per added inch of barrel. Likewise, you lose that same factor when the barrel is cut short.
Co2 is handy beyond belief, cheap, portable, not overly equipment dependent, but severely limited in cold climates. It's a great indoor gun where temperatures are stable. Further, this gas is self limiting in available psi. That's to say when evaporated in a closed cylinder it produces about 1400 psi, The shots will remain quite consistent in velocity and foot pounds of energy, also in number of shots from a 12 gram or larger gas container. Even with it's power limitations, it can get the job done within its' operating distances and templerature range.
HPA is where the power lies. But, you need a high pressure air tank, a fill station (hose, valve, pressure gauge/gauges, QD fitting, etc...)
The real question is how far are you going to shoot, and for what purpose. If it's Summer time fun and target, or short range pest control, Co2 is the way to go.
However, ranges over 30 meters, in various seasons and temperatures, you need HPA (high pressure air) to get the job done. For best power, avoid short barrels. Personally I believe 12" is a good size because it can be powered up to take out medium size game, 'coons, armadillos, and small mammals.
Caliber isn't so critical if limited range/stable temperature, use is the norm. The faster fps in .177 can offset the heavier weight of a .22 pellet...meaning the foot pounds of energy on the target can be just about the same at close ranges...30 meters or less.
There's boatloads of information available here and on Youtube about modifying both types of air gun pistols. DonnyFL has a moderator suited to your needs for a reasonable price, or you can 3D print your own, just don't expect the same degree of sound reduction in similar sized moderators when using equal sized professionally produced vs. 3D printed moderators.
Knock yourself out. Just have some fun while learning the sport. Personally, I'd start with an inexpensive C02 gun and get some practical experience under my belt before going to the expense of HPA.
Hoot
But when I'm done, it should be about 12" long. Shoot 16gr around 600fps, and be quiet.
