It depends on how much you give decide to give it. You can say "my line has 10Mb up and 20Mb down, I want to share 50%". It's up to you.
The more the better.
And it's secure to run in Germany. There are many nodes there. It's not like TOR. All the traffic stays within I2P, so nobody will be knocking on your door because someone downloaded something over your IP like with TOR.
The code doesn't seem to define a fixed number. The number is relative to who you're connected with. Looking at the speeds of my peers, anything above 100KB/s is "fast". If all nodes in the tunnel are 1Gb/s, then the tunnel is 1Gb/s. Therefore, the more high speed nodes, the better the speeds.