Let me take a crack at a few of your questions.....
But first, "Welcome to the Forum!", and to the world of MB.

Use the "search" feature; there is a lot of great info already posted here. In particular is a thread on DIY reapirs which is W123 specific.
Yes, the ALDA may be "tweaked" ,but its best to monitor exhaust gas temps to avoid melting stuff.
There are three main areas of concern with the W123.
1) Rust is one. Check particularly around the jack points.
2) The Air Conditioning is another.
3) Poor instument illumination is the third one.

Mechanically, the cars are excellent, and many run hundreds of thousands of miles.
Parts availibilty is excellent---Fastlane is the sponsor of this forum. I have used them several times, and have always been pleased with the parts and with Phil's quick serice.
W123 are also plentiful in the junkyards--if you're into that.

The steering box has been extensively discussed--use the "search"
I drove a 240D 4 spd, and found nothing in the shifter throws, or the quality of the shift about which to complain.
Keep us posted on your adventure.