Recommendations for new audio system

[Big Worm 1]

Hi

I've just bought a VW Passat B6 which has the bog standard RCD 300 stereo in it, single CD and radio. It is the older version so it doesn't even support mp3.

I want to get a double din system that supports DVD, Sat Nav and mp3. Ideally i'd like the VW RNS510 as it will fit perfectly without any ugly fascia plates around it, but it is pretty expensive.

Can anyone out there tell me -

a) if the RNS510 is any good, and where is the best/cheapest place to get one from?

b) are there any decent alternatives that anyone can recommend?

Thanks