Depends on your budget, whether you want sat nav, OEM or not.
A lot of people have replaced the rcd300 with rcd510 or rns510 (with nav). When upgrading to 3rd gen vw headunits in pre 07 cars ,you might get battery drain, which can be overcome with a newer canbus as well. With 57plate you might be lucky.
Some guys are also running cheaper Chinese copies with no canbus issues, and some good feedback.
Search through the audio/satnav section.