Does it have built in storage? If so, how much?
No, but it does have one USB port so you can plug in a USB harddrive or thumbdrive. I understand that some people have connected up to 1TB drives to their S3100.
I have the BDP-S390, which is the previous model, and usually copy videos to a thumbdrive for viewing.
It's also DLNA compatible so you can use any DLNA server on your network to stream pictures, music or videos. Sony has a free DLNA server called HomeStream, which is based on the popular Serviio DLNA server.
One problem is that Sony players are not compatible with all combinations of video file and codec formats. Most of my videos play fine on my S390, a few don't. I don't care enough to figure out why.