I can't recall if it's the 1480 or 1432 that comes in two pieces.
It's been a while since I had one apart. Only advantage I can think of was the ability for more power and an easier install from MB for all the speakers.
There are some later ones that should just hook up, I've never looked into it due to the cost. You can have better electronics for less cash now. Most aftermarkets looks like they came from 2001 A Space Odyssey with all the blinking/flashing crap but you can sift through them to find nice ones. Older Pioneer Premier are nice, new Alpines with the Biolite display, some older Kenwood eXcelon stuff, etc.
If you're only trying to obtain CD, a changer is a viable option. Less work and probably less cost. The 400E I almost purchased had every option including CD changer as well as heated seats.
Why did they use 2ohm stuff? Same reason they used a common ground for the speakers and used that center fader... we will never know.
Here's what I'd do:
$150 quality used headunit
$75 small external amplifier with install kit
$150 speakers
You'll have a modern head unit with better FM reception, XM capacity, CDR capacity, MP3 perhaps, etc.
Head unit power sux, a lil external amp is WONDERFUL. I stole a lil 35x4 Precision Power for $45 shipped, an install kit shouldn't be too spendy for 8awg wire and RCAs. Then you have clean power, use the headunit for crossovers.
Speakers, this is debated for the W124s but I'd just replace the dash and rear fill. Rainbow makes some direct swaps so they'll cost a bit more, but are a great speaker. Aside from that, most others will fit with some dirty talk.