If you buy used, you really only HAVE to make sure it includes the CDs and the diagrams.
I, for one, am keeping my primer when I sell the CDs(which will be in a couple of months, most likely). You can order just a primer from the LEEWS site. The price you pay, however, should reflect the fact that you're not getting the primer.
I think a primer from LEEWS is something like $20.
I don't think the editions matter very much (if there even are different editions; there may not be). Regardless, if you buy the CDs, you're getting a newer version.
As for when to start using it, that depends. I guess PLS recommends doing them in August before you even start school. I can see that in some ways that'd be helpful, but in otherways, worthless (like a lot of PLS, IMO).
I'd say doing LEEWS early in the semester would be good. Most 1Ls don't start outlining very early on, mostly because until you're in classes for a couple of months, you don't really know how to discern what's really important for an outline. Finishing up LEEWS right before you being outlining would probably be the best idea.
For me, I didn't quite listen to all the CDs in a weekend (like you're supposed to). I spread them out. I started late September, and finshed them in mid-October. Then I started outlining, and was up-to-date on my outlines by late November. I simply added to my outlines as I had class. I was focusing on doing practice problems for about 3 weeks before finals, starting just after my final memo was due for Legal writing.
I think that worked pretty well for me. I had more than enough time to focus on my exam writing skills, in addition to getting a good command of the material. It was also helpful to practice *using* my outlines efficiently while doing problems.
This worked for me, and I highly recommend it. But remember that law school is all about trial and error, and what worked for me may not be what's best for you.
Oh, and when I decide to sell my study guides and LEEWS, I'll likely stick them up on Ebay. I'll post links to the auctions here when I put them up.