Hey dude... I'll answer these from MY OPINION only, it may differ from other peoples...
1. the Z1 has other features other than DVD playback, that make it more expensive. It also is HD based while the D series is Disc based. So if you want your maps loaded onto your unit VS. loading a DVD with the maps on that.
2. If one of your main interests is IPOD integration, then YES, both the D2 and the Z1 have had horrible reviews on that function. and waiting could be your best bet. (although I'm not completely sure if they've fixed that, other people will post on this thread and they have much more info than what I can provide)
3.The units pretty much last forever. Although I ONLY bought mine last week, I'm pretty sure it will be truckin for quite a while. (again, other people will have more info than me)
4. If you are referring to the screen tilt as the "mechanical parts" than that really depends on what type of car you are mounting it in. If your double din slot sits low on your dash, than the tilt can be quite functional and convenient since it's angled up towards you. However, if you double din slot is high on your dash you may have no need to tilt it at all.
I hope this helps, again, I'm no wizard but I do know a little bit. I also know that I'm very happy with my Z1 purchase, But I am a "top of the line" kinda guy, and the lower end units could work absolutely fine for what you are doing. It's a matter of taste and practicality to make your final decision. just make sure in either circumstance you KNOW who your seller is, there is a lot of scams going on with these units, especially online, so be careful.