Topic close to my heart, did my Economics sub-thesis on this very idea.
Percentage caps, such as P75 proposed, are good for financial viability but do nothing for competition. Interestingly the G14 propose a cap of 70% of turnover for all European clubs.
If an AFL style cap were implemented, it would have to be Europe wide, as non-capped leagues would draw the best talent away. South America wouldn't have the financial strength to lure significant players away from Europe, though some South Americans may think twice about moving to Europe.
DIPPER in terms of the EU, the European Commission have stated that they would accept restrictions on the labour market on the condition that it improves competitive balance of competitions, so a salary cap would be potentially acceptable.
If a cap were installed, it would require a new transfer system with no transfer fees. In my Economics thesis, I suggested an auction mechanism, similar to the way mobile phone spectrum is auctioned, so as to allocate players to clubs that most highly value a player under the constraint of the salary cap.