A LOT, if you want to do it well. For example, for our Spartan Hoplite unit, several custom models and multiple skins had to be made, more unique abilities than you can throw a javelin at had to be coded, and completely original 2D art also had to made. Then all of those elements had to be put ingame and beaten with an aspis until they worked correctly. It was the first thing ingame and it still isn't fully functioning because we keep thinking of cool new things to implement
And that's just one unit. Now imagine all of that, then replicate it towards EVERY unit, EVERY building (which are worse, because you'd have to make original buildup, idle and destruction animations), EVERY Power, EVERY hero, a new UI and more 2D art, likely new voices (though those are admittedly a small priority at this stage), and if you want to be even more accurate EVERY new map relevant to the setting.