126 SD questions

[1980sd]

Hi all! I currently have a 1980 SD which is a fantastic car that I'm really infatuated with. I'm actually thinking about getting another one but want an upgrade for a DD.

I like the simplicity of the 116 but the ACC and steering box have me concerned. I'm actually thinking about replacing my Caddy CTS with an old Benz and sometimes I have to drive about 1000 miles for work so dependability is important!

Sooo...

How do the 617 powered 126s do on the highway and do the AC systems actually work without spending a bunch of $$ on them. I'm in Ga. so AC is sometimes important!

Is there much diffrence in economy/power with the 603 engines to make having an SDS? I'm familiar with the TO and head issues so I'm leaning towards a 617.