Few notes:

• I wouldn't avoid any of the well-rated US based K40 ebay sellers - $80 cheaper is nothing to sneeze at. My one looks like it had been opened and checked by the seller before I got it (but def. not second hand).
• Remarkably almost everything in the K40 actually works. Traditionally people have been paranoid about the internal grounding being painted over. Worth checking
• At least consider safety glasses, especially as the stock K40 doesn't have an interlock to prevent laser firing while the lid is open! I thought https://www.amazon.com/gp/product/B07C7KZJ8F/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1 was a reasonable choice - and at $USD 30 they're cheap insurance. Hitting anywhere else leaves a small, but surprisingly annoying, burn spot. I only needed one to learn :)
• Make sure the cooling water is hooked up and running before running the laser! Check it every time!
• A cheap water flow switch is good insurance there (but play a little first...)
• Until you get a current meter stay under 30% on the digital panel. The actual laser power will vary from unit to unit, but 100% is far, far too much. BTW you don't need to actually add one physically if you're confident with electrical things - the metalwork is a absolute PITA. A normal digital meter SET TO CURRENT and temporarily inserted in series with the laser ground (to work out the relationship) is OK if you feel confident working around high voltages (or make sure you stand back!) - but redo it occasionally as the tube ages.
• Do not use the power receptacles on the rear panel! The ones on mine actually squeeze the water pump plug out while cutting - socket melting or pump stopping (destroying the tube!) seemed inevitable.