No, it won't interface with the XM weather service. There are some network services included (some free, some paid) but you'll need Internet access for those.
Weather, satellites maps, etc. are also available through other means in RR but they require Internet access.
As for antenna, there are two main variations of GMPC, one that comes with a GPS receiver and one that doesn't. The version that comes with a receiver is licensed via the receiver itself. You can install it on multiple PCs but it will only work when connected to a licensed receiver. The other version is activated by a license key and can typically be installed on only one PC. With this you can use practically any GPS receiver capable of NMEA.