I am sure other users will post more throughly after me, but here are some quick answers to a few of your questions:
1. Shotgun gauge is determined by the diameter of lead balls. Namely, how many lead balls of the same diameter as the bore it takes to weigh a pound. A 12 gauge shotgun is approximatly .73 caliber, and it takes twelve .73 diameter balls to make a pound.
2. To understand choke tubes you must understand choke. Choke is a slight narrowing at the end of a shotgun barrel that helps focus the shot pattern, kind of a like a nozzle on a garden hose. Cylinder bore is no choke, Improved cylinder is slight choke, Modified is medium choke, and Full is alot of choke.
A gun that has interchangeable chokes has about a three inch section at the end of the barrel (where the choke would be normally) counterbored and tapped. A choke tube is a cylindrical piece of metal that threads into the counterbored section and has the "nozzle."
3. Yes, you can use regular shot in a rifled barrel, but expect large amounts of fouling and terrible patterns.
4. There are different kinds of slugs.
The first type of slug popularized is the Foster slug, more commonly called a "rifled slug" due to the ridges that allow it to pass through a choke without damaging it. The rifling actually does nothing to stabilize the slug. Instead, the slug is built kind of like a shuttlecock, with the weight in the nose and a hollow base. These can be safely shot through a smoothbore gun provided you don't have one of those insanely tight turkey chokes.
The second type of slug is the Brenneke slug. This is basically an improved rifled slug. Instead of having a hollow base the Brenneke is solid and has a plastic wad affixed to the rear. This is acutally even more like a shuttlecock. Brenneke slugs penetrate more than a conventional rifled slug because they don't flatten out and break apart. The Brenneke retains the rifling and is safe to shoot in smoothbore shotguns.
The third type of slug is the sabot. This is basically a large rifle bullet encased in a plastic sleeve that falls off once the slug has left the barrel. These slugs can only be used in rifled slug barrels, although they give superior accuracy and ballisitics compared to the Rifled and Brenneke slugs.
There is one last kind of slug, which is a huge, solid slug resembling a semi-wadcutter pistol bullet that is the size of the bore it is intended to be fired in. These can only be shot in rifled barrels. However, these slugs are specialty ammunition that you would have to specifically seek out and order. In fact I probably shouldn't even have mentioned them.
5. Quality pump shotguns are actually suprisingly cheap, even new. The Remington 870 Express runs about $250, and the Mossberg 500 $200. The Maverick 88 (a budget Mossberg 500) can be found for less than that. I would seriously consider getting a new shotgun over a used, as the price differential is really small between new utilitarian pumps and used ones (like $50).
If you are looking for a fancier shotgug then buying used can be quite a value, but the basic pumps are nice and inexpensive.
But this leads to another question regarding the choke tubes. I know they're screwed into the muzzle to adjust the size of the shot pattern. But what advantages/disadvantages are there to having a small or large pattern? And what sort of jobs are more suited to large or small pattern sizes?
