Since Minecraft rarely uses more than one thread, consider possible waste when buying more than 2 cores. However this does not apply to mods and plugins which utilize their own threads - buy accordingly. Assume that most of the server's load will be on a single core. The main performance factor is without a doubt CPU clock speed. CPU cache sizes and RAM latency are also factors. There does not appear to be a disadvantage to using high-end desktop processors.(verify?)
It should be noted that these suggestion presume default settings in server.conf - i.e., a Draw Distance no higher than 10, and is also based on a new world; the bigger the world gets, especially if there's advanced redstone circuitry at work, the requirements - especially the RAM, or in the case of lots of redstone or mobs (in e.g. grinders), the CPU - increase significantly (to the point where a major world with several camps, redstone machinery, grinders, etcetera can easily more than double the requirements).
RAMDisks can considerably accelerate world saving/loading in a very populated server. NOTE: information is outdated. Windows 10 has been the Primary version of windows for 3 years. Use of server-side plugins can allow game to utilize up to 4 threads for superior performance. RAMdisks are dangerous because if the power is lost even momentarily, all recent data would likely be lost unless constant backups are taken.
Used mods only[]
Windows 7[]
*During tests plugins were not used!
Requirements
Players
CPU
RAM
SSD
HDD Backup
WAN Up
WAN Down
Notes
Minimum
10+
3.0+ GHz
4 GB+
120 GB empty space
1 TB backup space
10 Mbit/s
150 Mbit/s
Recommended
50+
3.5+ GHz
8 GB+
320 GB empty space
2 TB backup space
50 Mbit/s
200 Mbit/s
Optimal
100+
4.0+ GHz
16 GB+
560 GB empty space
10 TB backup space
1 Gbit/s
1 Gbit/s
RAMDisks can considerably accelerate world saving/loading in a very populated server.