I almost bought a Octovia vRS but decided on an older Audi- think i made the right choice.
Home security is not an issue but work is a big one, I work as security at a big midlands Uni, and as you'd expect we know all about crime, there's alot of car thieves about. The only way you can combat it apart from removing the head unit is to leave your glove box open with nothing in. If that doesn't tell your would be thief that you've thought about removing valuables then nothing will! A big **** off sticker on the front windows saying "All valuables removed" would do well also, never seen any though. That is a point might see how much to have 100 made, could sell them for cost on here.
In answer to your music requirements, if i read you right you already have a sub, I'd go for a head unit that will take Mp3's on DVD's I've seen them for around £220, failing that just go for a mp3 cd player for sub £100. Depends on your needs really. I was looking at bottom of the range DVD screens for £400 but i believe that even though the face comes off, it still leaves and inch of the edge of the screen showing, which would be a tell tale sigh and would surely invite a thief in to your car. Broken windows are neither nice to find or cheap to replace.
Ok i've re-read your post. Like you i am a little wary about "loosing" that current Quality of the standard system. The way i look at it is like computers. I built my own. Different components from different manufacturers. There's no way my system will have ever been tested by anyone because it's one of a kind. In contrast you have Apple Macintosh. Computers sold as units with parts built for one machine! Anyhow someone else needs to answer this one for us both!It's late i'm tired and talking crap now and starting an 8 hour night shift!