The standard is 15 power to win and we have some cards that raises that amount (ex:North agendas) and others that lower the amount (ex: Treaties) but I am wondering if there is a lower limit. In other words what is the least amount of power I can have on my cards and still be able to win.

Honestly? 0 - but it's not easy to pull off.

If you get this version of Viserys Targaryen in play, get rid of all the other King characters in play, then put 15 attachments on him, you need 0 power to win.

So if your question is if there is a minimum number of power you need to win defined by the rules, the answer is "no." If you can stack up enough card effects to get yourself to needing 0 power to win, you win.

You answered my question perfectly. Thanks!

Does an opponent playing a treaty agenda limit Viserys or give him a head start?

Limit. The "minimum of 10" applies no matter how many power-to-win reductions you pile in.

The card imposes the minimum that the rules do not.