I have got an unused rPi and HDD and i would like to start a lightning full node.
A lightning node requires a full bitcoin node in order to run. Imagine that it actually sits on top of it. I would go with an SSD. The initial blockchain syncing process can be a pain using an HDD. Plus, the HDD will generally live less than the SSD.
Does anybody have already some experience?
Yes I have used two implementations for LN nodes. LND and CLN.
How much does you earn with a full node?
You earn sats only for routing payments through the network. Imagine you want to pay Alex who is in the US. In order for you to pay Alex, you need a route between your node and Alex's node. You can have a direct channel or an indirect channel, through my node for example. For the job of routing the payment I demand 1000 sats flat fee. Therefore, I will get paid 1000 sats. However, depending on multiple factors, your node may only process a very small amount of payments (or perhaps no payment at all). Finally, there are multiple approaches to payment routing. I mean, sometimes the shortest path is chosen, or sometimes the cheaper path is chosen. Sometimes the path is chosen randomly, assuring higher privacy but being more expensive.