Both the ACK800 and the DR-DC10 are relatively expensive (especially when you multiply the price by 40...), but as somebody else already mentioned, you find lots of cheaper alternatives from other vendors on Ebay or Amazon. Just search there for ACK800 or DR-DC10 or whatever the camera's manual mentions in it accesories section as an external power supply.
For a permanent setup on a long duration project like this, I would not recommend anything but a genuine Canon adapter. I have several of the cheap Chinese supplies ( $8 vs $50 for Canon at the time ) and I have had failures. So far its been the output capacitors, which I simply desolder and change. But that does not sound like something you are going to want to do even occasionally. Especially as the failure mode is for the power supply to appear to work until you either move the zoom lens or take a shot I also worry a bit about fire and make sure the adapters are not running when I am away and that they are positioned to melt down without affecting anything else.
Which camera to buy? (Any recommendations? Cheap, but OK quality)
Off hand, I'd go for something in the $150/110 euro range. The challenge with using CHDK is that the latest cameras (i.e. the ones in the stores) are usually not supported yet. In fact the most recent releases seem to have changed their firmware enough that it could be a long time before CHDK is ported to them - if ever. If you buy 40 cameras, its also likely you will get different firmware versions. This is managable but means the SD cards will not be all the same - the CHDK release has to match the firmware version. If you run into an unsupported firmware version let us know - there are a few people here who have become quite good at cloning new firmware versions from working ones.
From a photography point of view, does the size of the camera body matter to you? Also, shooting indoors I assume you will be mostly using wide angle ? The specs on each camera vary - make sure the one you pick covers the focal range you need. Flash on or off ? On P&S cameras, the flash units are generally terrible. You can mount external flash on the better cameras but that continues to get expensive. Otherwise, read the what the reviews say about "low light" performance.
As a baseline, I'd suggest you look at the A2300 (supported by CHDK and still available in store for $120) and compare other choices to it. If money was no object and you want the best possible image quality, both the G15 and G1X are well supported and readily available in stores. But they will run around $450.
A remark about the Eye-Fi and similiar devices: Having the ability to automatically upload the images via WLAN sounds nice and convenient, but I would not be surprised if the WLAN connection breaks if the camera is more that 10m or 20m away from the acces point: An SD card is simply not big enough to include a decent antenna, and metal parts of the camera's case may shield the radio waves. So you might need to configure more than one AP.
I'll continue to play around with scripting multiple cameras and USB hubs for my own interest but the wifi cloud option sounds wonderful.
One additional thought you might want to add to your "to do" list. You can configure CHDK to automatically run a script when it powers up. This can be very useful for setting things like zoom position to a preselected value. Otherwise you may have to run around to all 40 cameras doing the final changes that the default Canon firmware does not save between session every time you turn the system on.
