As someone who routinely hosts Minecraft servers and Undead servers, getting a 16GB RAM system with atleast a quad core (in the case of minecraft, clock speed is cared about more), and an SSD will be the most beneficial. Also make sure your pc has a static IP and you port forward your server so your friends can play. Also, make sure you have a decent upload speed of atleast 10 megabits per second. I recommend connecting the server to your router through ethernet or WiFi 5GHz.
If you have a budget of ~$400 USD, Server Monkey has good machines that you can configure to your liking. Their prices seem fairly decent for the content they will give you. The Dell PowerEdge R510 is their cheapest Dell Server, and it may be what I purchase someday.