[maeln]

It is pretty easy to setup if you know the basic of administrating a GNU/Linux or BSD machine. It is better if you also know a bit about security on those system too (at least how to configure the firewall).

When it come to the legal part, it depends.

Being a exit-node can be very tricky. In some country, you will have to register has a telecommunication provider in order not to be considered liable for whatever comes out of your relay.

Being a guard-node (the "entry" node for tor client) is usually safe but can still create some trouble. For example, the virus WannaCry was using Tor to connect to its C&C servers. Due to this, some Tor guard node got seized by the French police because they saw WannaCry connect to the IP of those guard node and I guessed, decided that it was necessary to seize them for their investigations ...

But you can configure your node to never be chosen has a guard node and to be just a relay and not a exit node. The node will be the middle man between a guard and a exit node and that should be completely safe, unless you live in a country where technology to circumvent censorship are prohibited.