You could go i5 but you need to configure the stream settings of each camera because it eats a lot of process even you are only using 6 cameras. If you are only using 6 then go for srn-1673s or 873s. They come with a built-in poe switch port at the back and can support full 2mp@ all channels
I have the samsung SRN-1673S and so far its only been compatible with itself. My ip cameras do not work with it. Only the samsung ones. I verified that they are ONVIF and they still don't work. So unless your cameras are samsung it will be a hit or miss with the SRN-1673S