My understanding is that the "dash numbers" are used to denote changes in the model features. Many times the modifications to the model can be relatively minor, such as changes to some internal part so that it can be manufactured more easily. A 28-2 would be the second modification to the Model 28.
The model 28 is basically a plain-Jane model 27 (the first .357 Magnum). They reduced the cost of the gun by getting rid of the checkering on the barrel rib and the topstrap, along with a couple other fancy features. The gun itself is a beast. When the N-frame .357's came out, they were typically being used with 158-grain bullets at screaming high velocities, and they were built to take a lot of punishment. Consequently, you should have less problems with end-shake than you would on a K-frame. If you have a gun that has a reasonably tight cylinder-to-frame fit, the only real concern you have left is the timing. As you slowly pull the trigger in double-action on a Smith, the cylinder latch should lock into the cylinder notch a hair before the hammer drops. In single action, the cylinder lock should engage before the hammer locks at full cock. You have to check all 6 chambers in both single & double action to make sure the timing is on.
Most of these guns were built a while back, before computer-controlled equipment was available, so every once in a great while, you may get one that was out on the edge of the quality control curve. I had one J-frame that had one chamber which was slightly off and would shave lead. If the indexing error is not too significant, a gunsmith can fix it by slightly reaming the forcing cone. Doesn't happen very often, but you can check for this if you get to shoot the gun before you buy. Put a piece of paper about 3-4 inches from the side of the gun (behind the muzzle but next to the barrel/cylinder gap, and you have to do it on both sides.) When you fire the gun, you will get powder residue on the paper and this is normal. If the paper shreds in any way, or if lead builds up on one side of the frame near the forcing cone, the gun is shaving lead and you don't want it, cuz it may be hard to fix.
) so would be interested in what changes occured in what years/ECO revs for that model, too. Eg, if I found a 27 made in '85 would it likely be post-P&R period, too?