The CZ-82 (actual designation vz 82) and the CZ-83 are for all intents and purposes the same gun with a few caveats:
1.) The CZ-82 was produced only in 9x18 Makarov caliber. The CZ-83 was produced in 9x18 Mak and .380 ACP (i.e. 9x17). The 9x18 Mak caliber is slightly more powerful than the .380 ACP, however on the other hand there is a somewhat wider range of western production ammo for the .380. This is slowly changing however as the 9x18 is becoming somewhat more popular in the U.S..
2.) As you have noticed, the CZ-83 was produced in a higher quality finish. The CZ-82 was only produced in a black polymer finish and was intended for Czech police and military. The CZ-83 was produced for the commerical market and was made in a variety of finishes (blued, nickel, polymer, ).
3.) The trigger guards are slightly different. The CZ-82 trigger guard was designed to accomodate a gloved hand and is therefore slightly enlongated.
I would say that in excess of 90% of the parts are interchangeable between the two pistols. As stated, the trigger guards are slightly different and I believe the slide stop springs are different (Norm Sutton can give you the details on that).
As to your other questions, here are my opinions:
1.) I don't view the absence of a decocker as an issue. Personally I've never gotten used to trusting a decocker to drop the hammer without firing a round. Some decockers can malfunction due to component wear, therefore I've always had the habit of manually dropping the hammer with my thumb.
2.) SA/DA means that the gun can be carried in Condition 1 (Cocked and Locked) and Condition 2 (round chambered, hammer down). The advantage of the SA/DA mechanism is that in condition 2 you don't have to manually cock the hammer with your thumb, instead you can simply pull the trigger which will cock the hammer and then fire the weapon. After the first round is fired the gun will cycle normally with the hammer getting set in the cocked position and the end of each cycle. Many people perfer to carry in condition 2 as they consider it to be safer (though of course opinions vary on this point).