So I'm just getting back in to this game and was playing a free game online against the bot, and I made a buy phase megaturn engine...
Man, things have changed since I last played this game.
Anyway, the board had chapel (to get rid of everything), inventor (for the megaturn), scepter (for the buy-phase engine), market (for +buy), smithy (for draw). But no other source of +actions besides scepter and acting troupe. The megaturn ended up being one where I play scepter (on my one inventor) to gain another scepter, do this three times, play scepter (to play smithy) and draw up the scepters I just gained, repeat for more scepters/inventors... It ended up being a 5-province turn.
Perhaps there was a more conventional engine here, it was my first time playing with acting troupe and scepter so I wasn't quite sure how to put those things together. But I figured I only needed to 'village' enough to get one copy of a few different actions in play, and from then on scepter could be every other card in the engine, and all during the buy phase.