What are additional IPs good for? My ISP sells additional (public) IPs for 3,60 euro/month, would it be worth it or necessary purchasing such an additional service?
For the size node presently indicated, additional IP address are of no particular value.
Extra IPs can be use for
1) running additional 'tor' router instances on medium-to-large multi-core servers with high available bandwidth (high as in 100TB/month per core for non-AES-NI CPU cores, multiple 1G connections or a 10G network connection for AES-NI CPUs)
2) allowing exit traffic to travel out on different interfaces than the OR port--not sure if that provides extra value
3) perhaps running a bridge node instance along with the router instance--is best to have non-adjacent IPs
4) for bridge-only setups extra IPs allow one to start new bridge instances when old ones are blocked by censors and/or simply run multiple bridges; again non-contiguous addresses are best
If you can obtain a "bare-metal," "root" or "dedicated" server (synonyms) for a good price, in general the router will be dramatically faster due to better latency afforded by the lack of a virtualization layer and lack of competition from other users. Similar for Linux container setups where virtualization is almost free, but servers can be heavily loaded with numerous customers.