ToME: the Tales of Maj'Eyal

In most open source games, there's that one really big addon that everybody knows of. Once you play through the original game a couple times you play through this really big addon that changes "everything". Is there any huge overhaul addon or something like that for TOME?

So I have TOME downloaded and I know how to program in LUA, but how do you develop on T4? I can't find a download link for the engine. Is it in TOME?

The version on the website appears to be free, but the steam version costs money. Why?