HI yall, I'm new to the MB world and to this forum. I've always loved the look of the W123"s. that being said I just picked up a 1982 300D with 184xxx miles on the clock. I sold my wife on this by telling her how great the mileage was (I drive 500 miles a week)and we could get the kids in the back easier (i drive a 2005 subaru wrx). the car checked out good, good compression, mostly rust free ect. So long story short I had a few qustiond I/d like to ask you guys if you dont mind.

1st when driving down the hwy at 60-70 mph i get a repeating surge. Ive changed the filters but not the tank screen (to do list)
2nd A/C compressor dont switch on at all. Ive bypassed the thing-a-ma-do behind the glove box and still nothing.
3rd pass side windows dont work. Ive rebuilt the switch and tried the drivers side switch on the PS. haven't ran 12v to the window motors yet. (to do list)
Lastly like i said i drive 500 miles a week on the hwy ( I-75 in florida) and i need or should i say have to keep up with traffic (80 mph). So tach dont work, will the 617 turbo run that kind of reve or should i be looking to swap rear diffs? I know the 85 had a 2.88? Do I need to swap the whole diff or just the internals.
I know there is more but i haven't found it yet. Thanks in advance for any help yall could throw my way.