OC619
I agree ADS-B is a much better solution, but if you look around countries that are playing with ADS-B, the favourite vehicle for carrying the data is the Mode S extended squitter (a sort of data channel within the Mode S protocol).
Unfortunately, there are various reasons why installing a current model Mode S transponder isn't very likely to work with ADS-B kit if/when it does arrive in Europe.
So, I don't think ADS-B is going to give us any kind of amazingly cheap TCAS, especially as TCAS installations are gradually falling in price. Currently, an Avidyne 600 system with top and bottom antennae is of the order of £10k-£20k depending how anally retentive the installer is about certification of every rivet that's needed to carry the extra avionics tray which has to be fitted somewhere, etc. You could buy the parts from the USA for probably around $10k i.e. the cost of a basic IFR GPS.