To address your other questions: I would be surprised if you found a *cracked* used slide, but the best value used pistols will be police turn-ins which will have a holster-worn slide. On here, you might not get as much holster wear, but you'll pay more. Sellers here are in my experience mostly reputable, especially guys with an online reputation to protect. I'd be more leery of a low count/new poster who doesn't seem to have an online presence at any of the big gun websites.
I like LWD's slides for the money, but you may want to get it coated, since they are normally bare stainless. Stainless is much more corrosion resistant than blue steel, but not as corrosion resistant as the OEM tennifer. Once it's coated, IMO it's better than OEM, but you aren't getting it cheap by the time you get parts and the coating cost. That assumes too you know (or are wiling to learn) how to install the parts yourself. A gunsmith would add more to the coast.)
So for the "wisdom" of choosing a custom slide, it returns to the question people asked you above- Why? If it's cost/value, the OEM glock is pretty well optimized in that direction.
If you want to tweak/personalize/improve your pistol, the first dollars/effort you put into it are better put towards improving the trigger. After that, some people like the grip monkeyed with. Personally, I like the grip the way it is, with the exception of a grippier texture- So grip tape or sticky grip panels gets my next Glock Improvement Dollars.
There's really nothing "wrong" with the OEM slide, so that's the lowest value mod in my book... I just do it because I like the custom touches on the LWD slide, and sometimes I like to do goofy things like put longer slides on than the frames came with, etc. Then you sell off the OEM slide to recoupe some of the cost.
