Thank you for the support!
You need a dedicated linux rootserver with 64 GB RAM and a 200 GB SSD.
It is recommended, to have a backup server. As I'm producing a new block every second day, I do not need a backup server yet. In case of failure, I will disable my witness for maintenance.
You should have some linux knowledge and know how to compile something.
It is important to monitor the server, as, on failure, the missing block count raises.
A witness should also update its price feed regularly, based on the STEEM/BTC price from exchanges.
Besides the hardware and the knowledge, a steem witness should contribute something to the steem community.
Thanks for quick and detailed answer! Being a developer myself, i'm interested in that kind of stuff.
Good luck for your vote climbing!