The front speakers are only 3" speakers I believe. Watch out for Crutchfield. They tend to either say they have no speakers for the 123 or they will sell you what they think will fit, which theu don't and you wind upo returning them.
The front speakers should not have tweeters that protrude above the top of the speaker or the covers won't fit back on.
I mounted an aluminum plate just in front of the spare tire cover area, attached to it is my 12-CD Pioneer player, driven from a Pioneer head unit of course. The DEH-series Pioneer radios are quite nice with XM satellite and some MP3 units available now. Installing the radio is not too difficult.

I use a A/D/S/ power plate under the wood floor in the rear (the removable floor that comes with the NON-third seat version car. If you have the third seat I don't believe its recommendable that you use a power amplifier under there because the wires would get caught up in the process of lowering or raising that third seat. Look for a old A/D/S/ unit they are very reliable for this application. Forget those huge amps on the market now, you don't need a LOT of power, unless you have a tote box size subwoofer, and it should use a separate amp anyhow. You can mount the amp for the sub woofer on the sub box itself.

As for speakers, I have a set of small monitors made for use in a home, mounted in the back of the wagon. They are wood, not plastic. Search for bookshelf speakers or mini monitors on eBay there are many that would work well in the back of your wagon and actually sound quite excellent as long as you have 20 or 30 Watts to drive them.

There is no way the rear speakers in the 300TD will ever sound good.
PM me for more details, someday I will take pictures of the setup.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body. '87 300D w/ Sportline suspension. No more '79 300TD w/ 617.952 engine at 367,750, it was my annual driver to Texas and I sure don't miss it poor A/C heater system!