The K40 can be upgraded in a eternity, here we have some articles focusing on the most important upgrades and fixes you can do for the machine.
The most common upgrades are on optics and exhaust. More upgrades which tend to be made after using the K40 a bit is more on the gantry, moving parts to make the K40 more reliable and exact for better work.
Our articles focus on everything from upgrades you should do to upgrades you can do. You do not need all upgrades, it all depends on what work you do with your K40 laser.