I received a nice PM for sharing my hints on this project we did on our 1976 450sl, so I thought I'd share as a searchable post. We love the new system, it looks good and functions well. I've gotten more than my share of tips here, so trying to pay it forward.
Components purchased - Boston Acoustics SE45 4" speakers ($79.99) and the Retrosound Model 2 ($329.00 - top end because we wanted the bluetooth - other models are less expensive). We wanted the retro look and it doesn't look completely out of place.
Installation hints - use the Becker stereo mounting plate that uses the two bars with bolts. The old receiver comes out and the Retrosound unit fits perfectly in it - just like the face plate provided. The black side mounting bars/panels are too large to fit in the dash space. I bent those into a U shape to fit, to use them for mounting the knobs.
Wiring hint: I had the unit heat up excessively the first time I tested it. I did two fixes simultaneously, so I'm not sure which fixed it, but both are recommended anyway. The Retrosound comes wired for front and rear speakers. I may have shorted the rears by accident and ensured that didn't happen again by separating and covering them carefully. Also advisable is adjusting the fader all the way to the front.
Spares hint: Be sure to have spares of the old style fuses. One of the small fuses is bound to blow. I did it as did the person who PM'd me.
Major frustration aside while I learned these things, it was a successful project that added enjoyment to the car as well as the safety of bluetooth phone and one touch play list advance. We installed on a weekend, so tech support was closed, but I have nothing but praise for customer service depts of Crutchfield (Retrosound distributor - $70 cheaper than direct) and Retrosound (who sent the pinstripe inserts that go behind the knobs as they weren't in the kit from Crutchfield).