At those distances, I would not be doing analog video, and definitely not unbalanced audio.
If you only have component outs to work with, it is possible to convert to SDI and run over the existing coax. More cost effective (since you mention there are cat5 in place) would be to convert component to HDMI and run over two cat5 wires using HDMI over CAT5 extenders. Component to HDMI converters range in price from $40 (Monoprice) to $800 (DVDO iScan Duo) to thousands of dollars for broadcast equipment.
For HDMI over CAT5 extenders, I recommend Gefen, but any reputable brand (e.g. Monoprice) should work fine.
As for audio, at minimum, run the unbalanced audio output into a DI box (active preferred if you will not be feeding a mixer downstream) and send the audio feed as balanced lines over the cat5. You only need a pair out of the 4 on a particular cat5 cable for balanced audio. 300ft of balanced analog audio over cat5e is routinely done (I do it) without noise or hum.
-JR