Can't really help you out with the FM modulator. I'm going to bypass the head unit completely so I haven't looked into any of those. You can click here to see a listing of search results for FM modulators on www.mp3car.com. That should be a great resource for what you're looking for. One thing that I've picked up on is that you should try to get one that hooks directly into your antennae.

I'm not too worried about hard drive reliability during wheeling or going over bumps. I've toted around my MP3 brick of a player with a notebook hard drive (which is what I currently use) and I've beaten that thing up worse than anything my PC would go though short of an accident. It took it and kept on going. I'm not sure about desktop drives though. You might want to look at some of the threads on mp3car to get an idea of what others with full sized PC drives are doing.

Please do show pictures once you get it all together! I'll be doing the same.