1. not really sure on the 3.5's being better in cold.. try reading up on the cold temp specs on the drives you have available-- some might be better then others. maybe you could load everything on one hdd, and then copy it to other sizes, and then you do have some backups in case the one drive doesn't work correctly in the cold.
2. i go along with the theory that a smaller platter drive is going to be more durable, though there isn't too much to back that up..
3. the pico psu will not work without a lot of extra parts.. it will only accept a 12v input. cars electrical systems put out anywhere from 5-15 volts..
if i remember right, part of the pico-psu's issue is that it does not moderate the 12v line, so if your car is putting out 5 v, that is what the 12v line is at--which could shut off/ burn out components, where 15v could also do the same..