Mob Spawn Modifiers

Hi guys, I'm new to Minecraft modding, so I wanted to see how feasible what I wanted to do is and get an idea of the best direction to get started.

The general idea is to add higher tier mobs to create more of a challenge to help offset the armor and tools that many of the popular mods add, but I don't want to sick them on the players too soon so they can't establish themselves. So I want to limit their spawns to a minimum radius from the map spawn, and possibly also add a block or structure that also creates it's own radius. I'm guessing there isn't any existing hooks for this type of spawn mechanics, so is this something that can be added without a prohibitive level of effort, and can it be done without turning it into a messy jar mod? (Not a deal breaker, but I'd like to keep to best practices if I can.)