i bought mine around a month ago...the gti does really put a smile on your face...especially when you hear that dsg fart...look out for the timing chain tensioner...find out if the owners have had it replaced..as this is a common fault in the 00-10 model...you can tell if its on its way out by starting the car...you will hear a sort of grinding noise..theres a thread on here about the issue..and also a video of the noise..
About the mpg...35 mpg is probably achievable if you drive very sinsibly...which at times is hard haha..
to be honest i hardly use my satnav...i prefer using google maps..ive used it maybe once or twice and it is good