You can certainly use CHDK for this sort of thing, it can have scripts set up for timelapse very readily. It works with most Canon camera that have a Digic 2,3 or 4 chipset, which is pretty any of them that use SD cards and aren't DSLRs.

You would need a reasonably modern one, that took SDHC cards, otherwise it would have a limitation of 2GB, which might be a problem.

Power shouldn't be too much of an issue. They all use a single cell lithium-ion battery, which is nominally 3.7v. They're generally rated at 4.2 to 4.3v. Assuming you don't have any mains electricity in the area you want to use the camera, the easiest method is probably something like a small 6-12V SLA battery, around the 7Ah range is very common, running a 4V switch mode power supply.

The quickest and simplest way of doing this with off the shelf parts that I can think of is:

1 x 12V 7Ah lead acid gel cell such as this (it's overpriced, find a cheaper one!)

1-4 diodes such as these

1 x RC model S-BEC such as this (this will give you 5V with 90%+ efficiency)

Wire the SBEC to the 12V battery, and you have a nice little portable 5V power supply. Use the diodes in series with the 5V line to drop enough voltage to get you into the 4.2V range the camera requires. In theory one will do it, but as the voltage drop of diodes goes down as the current goes up, you might need two in series. Use something like a 6V flashlight bulb as a load to draw about 1 amp, and make sure it doesn't go over 4.3v like this. The camera will work down to about 3V or so, it will just give low battery warnings.

You wire the power supply to the battery terminals in the camera, and run the wires through the little covered hole in the battery compartment hatch that almost all of them have, even though they never produced a battery eliminator that fits the damn things!. Put a switch and fuse, around 1.5A, in line as well.

The whole gubbins can be packed into quite a small discrete box that no-one will notice, and given a sufficiently large memory card will snap pictures for days with no problems.

You could obviously use different batteries, or power supplies, but this is all off the shelf bits that shouldn't cost more than perhaps $40. Plus the camera of course.

pca