Unless you're buying an older model S&W, I'm not totally sure they still make the square butt models. I **think** they're all round butts now.
Yes, the model 19 & 586 are two different frame sizes. The "J" frames are the little 5 shot snubnose models, the "K" frames are the model 19, 64, 65, 66, etc. "L" frames have the full-lugged barrel such as a 586, 681, etc. "N" frames are the models 29 (44mag), 25, 629, etc.
Go get a copy of Gun List newspaper at a bookstore such as Barnes & Noble & you'll find a bunch of S&Ws listed at various prices. I buy almost all of my S&Ws used in order to save money.
As for timing & cylinder play/shake, it depends on the loads you use rather than specific calibers. 4000-5000 rds w/o any major problems aren't unheard of unless one is shooting maximum/hunting loads almost all of the time.
Hope this helps.