Reversed King of Wands: The reversed King of Wands may indicate that you may be tempted to accept the opportunity that presents itself, but it is best to make a thoughtful assessment before making a decision. There may be obstacles or complications associated with this opportunity that are not yet visible.
Ten of Cups: The Ten of Cups can mean that if you decide to decline the opportunity, choosing not to consider the gold coin, you may not realize the future opportunities that may arise. While that seems like a tough challenge at the moment, there could be big gains in the long run.
Ultimately, it seems like you need to make an educated decision about accepting or rejecting the gold coin. If you decide to proceed with caution and examine the opportunity more closely, there is a good chance that it represents a sign of luck for you. So, the answer to this question should be: Yes, I believe finding an old gold coin in the attic is a sign of luck.