How would the acceleration on a 1973 3.0CSi w/auto?
(I have an '88 535isa now for reference)

Not night and day different; depends on the differential ratio you're running. Cruise rpm is higher on the 3.0 due to the 3 speed tranny, though.

Are sunroof cars worse for rust? (I heard that years ago, but not sure if it is true).
The sunroof and the rear window wells drain into the rear rocker interior. More water, more rust. Also, condensation in roof tends to rust the sunroof area from inside out.

What tire sizes fit well on the stock 5 spoke alloys?

6"X14" standard wheels accept 195/70-14 (stock size), but you can go 205/70-14*, or even 225/60-14 or 235/60-14* (though the last two are pushing it).

Bob
* These will give a 5% low speedo reading

Thanks,
I think I may go look at one tomorrow.
Michael

Remember the three things to look for.

They ARE beautiful cars, though, aren't they?