For HUs, have a gander over to caraudiodirect. HUs with SatNav range in price from £400 to £1400.
IIRC, VW HUs restrict output to prevent speaker damage, so a new HU and new speakers will give a much better sound.
A few people here have simply upgraded to VW RNS-510, including me - mainly for looks/integration rather than features. Others have gone for aftermarket HUs.