the culture is third legion but i think to change it you need either 300,000 or 500,000 denarii to do it

It does take quite a bit of coin to change culture to Venetor. Additionally, you had to have helped Venetor re-imerge as a power through the quest. Lastly, some cultures do not unlock until you have reached a certain character level or relations with a particular faction. Such as Ukunda, Falcon and Venetor. They are rarely available initially as options for your own realm's culture, regardless of how much coin you have.