Hey @lukas - I principal there is not minimum for anything. However - we set by default a very high gas limit for all transactions. You will only pay the gas that is actually used but to create the transaction you need the ETH you want to spend + the ETH that is reserved for the gas (only a small fraction of it will be used).
What we will do in the future is a better estimation of gas costs for the transaction so that this amount will be smaller.
For now I think you are fine with 0.5 ETH. And then buy some shares for 0.4 ETH e.g. - this should work.If you just want to test it you can also do it here with test-net Ether: http://test.groupgnosis.com/
You can get free test net Ether here: test.ether.camp