All the data you want can be found at the hard drive manufacturers' web sites.
1) Seek Mode consumes the most power (see my link at the bottom). Idle power and below would be very accurate. Seek power would vary slightly from the listed value depending how intensely the hard drive is reading or writing so the example I gave should vary between 0.85 and 2.5 Watts. I estimate that this hard drive would consume about 1.5 Watts on average when recording a TV show.
2) Read the specs at manufacturers sites. They list the power consumption for every drive.
I'm not quite sure why you are so concerned with getting the absolute lowest power in a 2.5" drive. A really efficient computer would use about 40 Watts. Most use closer to 100 Watts. This drive uses 2.5 Watts maximum and 0.85 Watts when idling.
My recommendation is one of the 2.5" "Western Digital Scorpio Blue" drives between 320GB and 640GB. The consume a maximum of 2.5W
Here's the link:
http://www.wdc.com/en/products/products.asp?driveid=506