A D5000 has a built-in intervalometer that can take 999x3 exposures - total of 999 exposures bracketed 3 times each if necessary. If you want to go cheaper, you might find a Nikon D2x which is weather proof. If you are doing a 24/7 time lapse you might have to check back every few days. If you want to stick to Canon, you will need an intervalometer (the one Canon makes only works on the high end models) and the same process applies.
Another option is the Sniper Pro cam but it is purely auto - but very cheap and can run for months. Here's the link:
http://www.stealthcam.net/HTML/sc_sniper_pro.htmlIf you are going for solar power or a non-weatherproof system, you will need to build an enclosure. A fibre glass enclosure is best (like the harbortronics system), and it's not very expensive to fabricate.
Avoid metal for
1. Static
2. Heat
3. Rust
4. Attracts lightning (even though a fully enclosed system might protect the camera even if struck, but you will need solid earthing technique)
Avoid wood for
1. Insects and worms
2. Decay and mould
3. Used by birds
4. Not weather proof, can leak water
Hope this helps.