I got an 2012 rcd510DAB out of a new car that got upgraded to rhs510, from turbo herbie on ebay for about £220 delivered.
the sd card works really well.
if your car is 2008 onwards you will be ok with battery drain, i think your can gateway needs to be revK or above on pre 2007 cars.
some guys on vwvortex.com have taken a switched feed across from fuse box, looks a bit messy.....