You'll get a huge voltage drop running 12/24v down CAT5 for such long runs. Not sure on the diameter of CAT5, but at a guess I'd say it would drop at least 5 volts 150m down the line.

Btw as for the wiring, you have the right to ask them to do it for you. Unless you have the right quals its illegal for you to touch anything on the supply side of a FCU.

With 3 phase unless you're going to other buildings you shouldn't be too far from the right phase (obviously subject to a LOT).

CAT5 Would in my estimation (calcs to follow-not) drop too much V.

http://www.seattlewireless.net/index.cgi/JppsHomeBrewPoe

Anyhow always best keeping the power apart from data runs.

HTH - Peace