(Thinks: how would the manufacturer expect the user to add a nic?)
SFF-PC's are not designed to be very expandable. Though, usually, the pci-capable versions have an adapter allowing normal sized pci cards to sit parallel to the motherboard. (Depending on the configuration.) However, some are based of laptop technilogy - using mini-pci cards and pcmcia cards. Are you sure this isn't one of those?
Supported wireless - go for prism chipsets.
As usual - work out what is available, then look them up.
Sometimes this functionality is intended via usb...