It shouldn't matter if you gave Willy any gold while in Broken Valley, or got him to give you gold in Aleroth.
It also doesn't matter if you tell him you will help, or tell him you will not, when he is outside his house. When you follow him in (despite mentioning a key, he has to enter the house first, at which point you can follow), that is when you make the final decision whether to help him or side with the assassins.
If you side with Willy, IIRC you can not get back into the house after you leave (at least if you tell him to run when he tries to sell you the house). If you side with the assassins you can trade with one of them, so I assume you should be able to exit the house and return (I tried the quest both ways, but don't recall if I left). The Shadow Archer Cuirass has better stats if dropped when you kill the head assassin than if you buy it from him, and he wasn't that great of a merchant anyway, so I sided with Willy in that quest (though didn't end up using that set at all).
