Mark,
With regards to burning, the higher the wattage, the quicker it can cut, thus resulting in less burning. With the Mini 18, I upgrade to the 40 watt from the 30 watt for a bit more speed and power. The Mini 24 can go all the way to 60 watts I believe, but that was out of my price range. Six ounce leather or less has little to no burn marks on my machine.
As far as software goes, Epilog is awesome. For engraving, it acts like a printer. You can use any software (Word, Publisher, Photoshop, etc) and with the driver, it engraves the image. For cutting you need a vector program. For $200 I purchased Corel Draw, and it is perfect. It is easy to use with a bit of practice.
The Zing series is the cheapest, starting at $7995. The Mini series is more industrial and quicker. I opted for the Mini due to this. The Mini 18 (40 watt) was just under $12k. I chose to get a Bofa filtration system so I didn't have to exhaust outside (about $2200). They offer an air assist pump for $300, but I just used my compressor since I had one. You only need 30 psi.
As far as wattage you need, I think the 40 to 50 watt option would be good. I would also recommend the Mini verses the Zing. I would have loved to have the 24" model, but it pushed my budget.
The detail that I can do on this laser is incredible! Attached are two images of some more leather work done on the laser. The wallet was engraved, cut out, and I even had the laser cut the holes (I love it!).
Let me know if you have more questions.
Thanks,
Karl