Need guidance how to make a renaissance settler replacement

So the idea is simple, a unit unlocked somewhere in the renaissance that replaces the settler (like the swordsman replaces a warrior).

This unit ("The Colonist") will settle cities with some extra pop, instant buildings (say a monument and a granary) and possibly some extra territory (like the shoshone ability, though toned down).

I've looked around the modding wiki and couldent find anything to help me get started with this.

I made a few small mods before, but they all involved mere XML and no lua or dll.

How do i approach this? can it be done? do i need to use lua or must it be done through DLL modding?

Original thread:
http://forums.civfanatics.com/showthread.php?t=519949

Thanks!

Have a look at Gazebo's CSD.
There are two new settler units that appear at certain techs that do exactly what you describe.

Also CEP did a similar thing with policies, making the settlers found advanced cities.