Install Help Alpine CDA-9815 into 94 E420

[wjm]

Sorry.. reposting with better subject line......

Folks,

I'm planning on replacing the head unit in my 94 E420 (Becker 1692) with an Alpine CDA-9815.

I've contacted Becker USA and purchases a harness that connects into the current vehicle wiring so that I don't have to do any cutting. I plan to make becker to alpine harness using the harness I purchased plus the one that comes with the radio. I should be able to figure out the wiring using the following:
http://www.beckerautosound.com/1492.html

My questions:

Aside from making the new harness, is there ANYTHING ELSE that I'm missing?

Do I use the current Amp (assuming I have one)? If so, how do I wire it in? Or should I wire around it?

How about the current speakers; can I leave them in for now and upgrade later?

Will this head unit fit nicely, or do I need to retrofit?

Do I need an adapter for the antenna lead?

Lastly, the diagram in the link above seems to indicate leads for 4 speakers. .... yet I believe I have 6 (2 front, 2 door, 2 rear) right? If so, can someone explain the discreptancy?

Any help/comments would be appreciated.