* Free Democrats aim for kingmaker role in coalition
* Latest polls suggest 5-7 percent support
* Party would sell stakes in Deutsche Bahn, Deutsche Post
By Michael Nienaber and Thorsten Severin
BERLIN, March 31 (Reuters) - Germany's business-friendly Free Democrats (FDP) want to win back voters and become kingmaker in coalition talks after a federal election in six months by promising lower taxes, higher investment in infrastructure and a tough stance on Turkey.
Recent polls put support for the FDP, which was the junior coalition partner to Chancellor Angela Merkel's conservatives from 2009 to 2013, at 5 to 7 percent. That suggests it will get enough support to cross the 5 percent threshold to enter the Bundestag lower house of parliament in elections on Sept. 24.
Presenting the FDP's platform titled "No longer on the sidelines", party leader Christian Lindner said the FDP wanted to become the voice of the "impatient centre" of society by emphasising self-responsibility and liberal values.
"People can do great things if you just let them," Lindner told reporters, adding that the FDP's motto in the election campaign would be 'German courage' instead of 'German angst'.
The FDP wants to manage the balancing act of lowering taxes and increasing investment in education and digital infrastructure by privatising state assets.
Lindner said the government could raise over 10 billion euros by selling stakes in firms such as Deutsche Bahn, Deutsche Telekom and Deutsche Post.
On foreign policy, the FDP called for the European Union to quit membership talks with Turkey and freeze any further financial aid to the government of President Tayyip Erdogan.
"For us, Turkey in its current state under the despotic regime of Erdogan can no longer be a candidate for accession," FDP party secretary general Nicola Beer told reporters.
Lindner repeated his stance that Greece could only stay in the euro zone and receive more financial aid from its European partners if it implemented the structural reforms agreed in its bailout programme.
The likely alternative would be a debt cut, which would result in its departure from the euro zone, he said.
Lindner said he wanted the party to regain its traditional role as the decisive force in shaping a coalition government.
"I want to be the chancellor maker," he said.
Asked about possible coalition partners, Lindner only ruled out joining forces with the anti-immigrant Alternative for Germany (AfD) and the far-left Linke.
The FDP's power in state legislatures has been diminished in recent years by the rise of other parties, including the AfD. It failed to win enough votes for representation in the Bundestag lower house of parliament in 2013. (Editing by Julia Glover)