The 40w ebay laser's (40 watt, 40 w, K40, dk40, DCKIII, etc.) are essentially all the same. The laser has a 30 watt or so tube in it but that said 30 watts does a good job for a lot of the hobby work. The machines are more bang for the buck (if you get one for under 600 (700 with air assist) US delivered) than any other entry level you could buy. Research the before mentioned models on this site for horror stories to great successes. Study those that had problems and see if you think you could troubleshoot similar problems yourself. Lasers in general require advanced thinking skills (pretty much not what you get in high school today) and you will need to to be able to operate and understand a volt ohmmeter, and have decent mechanical skills. Service and warranty are virtually non existent and you should be the sort that blames yourself for any and all shortcomings in the machine after it's on your doorstep or you will find yourself complaining about "these are crap". These are NOT a consumer machine in spite of the ebay descriptions. You are getting something that in a good commercial model would run $15,000 or more. Keep about 200 as a reserve to purchase possible needed replacement parts. Search thoroughly on here, get the best deal you can, If your lucky it will run right out of the box, know the dangers, adjust the mirrors and laser on! These are great fun and quickly become your go to tool. You will quickly run out of room and want a bigger work area. 30 watts properly setup is actually fine for the hobby speeds we run at.
You have to add air assist immediately else the smoke will ruin your lens within a few minutes unless you are etching glass or another smoke free material. Smoke on the mirrors or lens causes them to heat and burn and crack. The weakest part of the whole machine is the software. My favorite of them is LaserDRW and it still misses the mark by quit a bit. See $32 laser controller thread for a possible arduino solution.
The diode lasers are much more dangerous than the C02 laser to your eyesight than the CO2. You can't focus the invisible CO2 beam and it takes a while to burn into your eye where the diode visible beams will focus on your retina and burn it out in an instant. No second chances. I would not want an exposed diode laser operating where my eyes are even with glasses since even reflections bouncing around the room can be as powerful.