Go as big as you can. I have the 6" inline duct fan on my 60W and love it.
Honestly when I think about the K40 I think with the "if it works, it's good enough" mentality. It is a small hobby machine with the intent for hobby work. If you are thinking about things like z axis adjust you may want to consider a different bigger machine that already has that capability. I believe the heads on K40's slide in and out of the enclosure so if you need to readjust your focus you just slide the head into the desired position to accommodate X material thickness. Of coarse this is only good up to a certain point. The head will only move so much before you are out of room.
The flow check gauge is a highly recommended add on. When it is wired into the water protect ports of the power supply the flow gauge will only allow the power supply to send power to the tube if the flow gauge says there is water flowing. It looked like this model was already equipped with water temp sensors. As far as ideal temperature, about 17-21C is considered ideal range for Co2 Lasers. Again if I had a K40 I wouldn't worry about it too much unless temps started getting above 30C. Higher water temp = less power at the laser, this is not mA power, this is raw laser output. Frozen water bottle in your water bucket will give you more working time. What you will need to worry about is contaminants. Closed loop is better to control contaminants, just change your water regularly and you should be fine, keep your water out of light to help prevent growth. Distilled water is recommended. I hear in extreme cases tap water can have enough impurities and conductivity to have power arch through and break a tube....plus promote growth of nasties in your water, that's the main problem. Gunk in your tube and hoses restricts water flow, causing your tube to heat up faster, reducing working time.
The ammeter looks to be similar to the standard ammeter many of us including myself use. I believe the rating in the corner means that it can be off by about 2.5% at a measuring of 100mA, a smaller rating would increase the accuracy. The main difference on yours is it doesn't have a little screw hole to adjust the needle position to 0 in the event it is not set properly or drifts away from 0 with use, not a big deal, the important thing is it actually has one. Many people use their lasers without knowing. I've checked my ammeter against the digital ammeter on my power supply and it looks like the numbers match up.
The compressor you are looking at is totally overkill. Cute little compressor to lug around the house for small jobs where compressed air is needed. Keep in mind it is a compressor and as it compresses it will hold some moisture from the air in the tank. It has a clear valve at the bottom that should be opened and vented to allow the trapped water to escape after every use. Allowing moisture to build up in the tank can cause issues to the compressor down the line. A moisture trap on the hose to the laser is up to you but can bring some piece of mind if you are worried about water splatter coming out of the air assist and onto the lens. I personally wouldn't worry and would rather have a spare lens on hand over a moisture trap. ANY air assist on a K40 is good. I personally still run my aquarium pump on my 80w OMTech laser. I have a little pancake 2.5 gallon compressor in the garage I use for other things and never think about adding it to my laser (for now, things can change depending on my application, or if my little aquarium compressor dies and I need to do a job right then and there).
As far as honeycomb replacement, my 80w machine came with slats and a honeycomb bed. I prefer the slats for most things, the honeycomb is good when working on very flexible things like paper, cloth, very thin wood that tends to bow a lot. Another option that you can make fairly easily is a nail bed. You cut a piece of wood to size and nail away rows until you have a nice surface you can lay things on nice and flat. This will allow good contact for material to be supported and minimal reflection marks from the laser hitting surfaces like on honeycomb. The wood supporting the nails might burn a little depending if the laser can actually make it down that far but it's a K40, "if it works, it's good enough." You can always add a 2nd layer of wood on top with bigger cutouts so it slides down the nails without resistance, that way when it does become charred beyond use you just throw it away and put another one on top to protect the main one with your nails attached.
For fans I here some people try these maybe not that size for a K40, maybe a 4" unless the K40 has space for bigger, but I tried using one of those. It worked pretty good venting straight from my laser, but it has no pressure when I tried to pull air from my filter box. It is fairly quiet, and if I wasn't using my filter box I would be using that fan now. It's been dedicated as my bathroom fan now and it does a good job of keeping the mirrors from fogging up during a shower and the humidity low to prevent mold growth (which was a problem before the fan was equipped into the bathroom). Any ducting would work, hard ducting with minimal bends and length would be best for airflow but your standard flex duct should be just fine and is good if your machine isn't going to be stationed at the same place forever. Makes it easy to move around, hard ducting would cause issues to move your machine even inches. Vent outside, use your machine in a well ventilated area, away from living spaces, like a garage if possible. You will still have fumes leaking into the space unless you have amazing industrial extraction.
From the pictures it looks like it has a drag chain on the X axis, I would hope it has one on the Y axis as well. It keeps wires and the air assist tubing safe.
Long, but I hope this helps.