1 - It's not overkill.
2 - I think the pre-assmbled unit is the better way to go. There are valid reasons to consider a Xenarc brand screen, but you're not going to get the easy fitment, either. And it wouldn't save you any coin.
3 - Not really.
4 - IG2009 is the cream of the crop for North America at this point and time.
5 - Amplifier.
You really ought to read the FAQs. Link in my sig, for your convenience.