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.
A TOR relay is pretty safe. You're just a middle-man and have no clue about what's going on.
Your IP is still going into every single blacklist of corporate gateways though (because F5, etc. don't care): so don't host multiple services on that IP/server.
An exit node is the most dangerous position to be in, because that's were all the bad stuff can be seen.
The GP didn't necessarily say he was running an exit relay, only that he was running a relay of some kind. As far as I know it's a lot less perilous to run a middle relay.
Hopefully it's easier now than it was then.
As others said relay nodes are safe and low risk to run. I wouldn't run an exit node without looking into the legal risk and having a plan.