If you are intent on investing regularly, then perhaps wait until you have accumulated at least €1,000 on each occassion. Then invest and the trading costs should be low enough in relation to your investment so that they don't impair the likely returns, particularly with the online stockbrokers like TD Waterhouse, FXCM Securities and even Davy.
Your expectation of return at 6-10% is a little wide. In the past, at the average values of about 13x earnings, equities in the larger developed markets have delivered 5% per annum over inflation. Today, inflation is circa 2-3% and markets, bar the US, look to be valued along long-term norms suggesting annual returns from here on a 5-10 year view of circa 6-8% before costs. Of course, the businessc cycle is alive and well and will deliver negative returns at times along the way. Your regular investing plan should allow you ride through the inevitable next downturn.