Are materials for potions, etc. replenishable? In other words, are they finite and not farmable like in Risen and similar games?

Mostly yeah. Some are rare and come from boss battles but plants are re-usable after a day or 2 and most of the other materials are refined from monsters which respawn.

The alcohol bases for potions don't respawn normally, and only a few respawn in merchant inventories. All of the other ingredients can either be farmed from monsters, or gathered from respawning plants. There's also MORE than enough alcoholic bases to last you the entire game. However, some alcohols will only support 3 or 4 ingredients, instead of the full 5. But you can also brew as much White Gull as you want, which supports the full 5. So it's not really an issue.

Thanks for the reply serpantino and bevinator! Just what I was hoping to hear :)