Model 715 is about the same size as a S&W "L" frame 686. As a matter of fact if you can't find a holster specifically for a small frame DW most holsters for S&W "L" frames work just fine.
Echoing the others, barrel changes are pretty staight forward, just take your time until your used to it.
As to quality of the older guns, look for those made in Monson, Mass. The Palmer, Mass gun quality was decent but towards the end it became hit or miss. Not saying they were all bad, but they did let a few clinkers out.
The Monson, Mass. guns were primarily made and shipped under the watchful eye of Dan Wesson when he owned the business. He was an absolute stickler for quality, particularly if his name was on it. Once it left his factory, (an old school house on Main Street), he didn't want to see it back. He was the same way with the injection molded parts I used to get from his molding company. "No money in making it twice" he used to say. I know this for a fact as Dan Wesson was a business associate and later a very good personal friend of mine. Each revolver that left the Monson facility was checked 100% for fit, function and most important trigger pull weight in both SA & DA mode. The older Monson guns will most often have much lighter triggers than the new DW's. They didn't have to keep lawyers at bay.
The DW Supermag Series developed around IHMSA matches and consistantly took top places. If you can find one of the older Monson one's in .357 Maximum, as I was fortunate enough to do last year, grab it. These revolvers were virtually hand made, custom fit and have beautiful actions. Mine is as smooth as silk.
You can indeed shoot .357 Mag and .38 special out of the .357 Max. Keep a piece of .357 Max brass available when you clean the cylinder. Insert it into each chamber and it will scrape out most of the fouling. This works best if you bell the case mouth ever so slightly so it's a push fit. Follow up with a good scrubbing and your good to go.
Sorry for being long winded.
Take Care