Just bought a lx3 black from lowyat forumer iamsobloodysick. However he did not provide me with receipt. Would that be a problem in the future in case of any warranty claims?

Guys, I'm a newbie in photography. This weekend is my brother's engagement. If I use the iA mode to take pictures both outdoor and during the dinner function would be sufficient? If got any other recommended settings please let me know. Thanks.