knx is the way forward as it is an open protocol and therefore there are over 130 manufactures of knx stuff.
as a installer myself i would advice you to rip out the cat5, as a knx installer would not use it because the cable we use is screened.
if you do wire it yourself you should idealy wire it in a radial format.
as previously mentioned you can talk to colin or scott at ivory egg they will give you some great advice and put you in contact with an integrator near you.
as for cost knx is about half the price of homeworks and is much more expandable just check out some of the stuff that you can have on the ivory egg web site like a 3.5 inch touch screen with 4 scene controls and 4 timer controls made by inzennio of spain also look at www.knxuk.org for more info on the system.