Sorry, but anything attached to the headrest area, or attempting to get 8's into the doors, has a decent probability of not looking really good. Just my opinion, not trying to insult anyone.
You can get the door panels from Jehnert with no speakers for $400 USD, and then buy whatever drivers you want. Their price including four Quart midbasses is still decent though. It gets more expensive if you want the full kit with the dash adapter, mid, tweeter, and 3-way custom crossover assembly (that's $900 USD shipped). It will be nearly impossible to put anything up front without massive work and/or custom fabrication, and at that point, you have to consider what your time is worth.
Putting a sub or two in the rear deck is MUCH more feasible, and can be made to look stock without too much trouble or cost. I've seen some decent free-air installs in a 124. They won't pound that hard but they can look great and give a nice bottom end. For maximum / ultimate bass you need a bandpass box in the trunk, that vents into the passenger compartment... and lots of power!
