There is a good deal of subjectivity involved in vintage watch collecting. Some collectors are very critical, and demand originality and a high standard of condition. Most are more lenient, and are comfortable with appropriate replacement parts, etc.
Case polishing and the refinishing or replacement of dials are polarizing issues; some have little or no tolerance, while others are relatively relaxed about them.
While I am not a perfectionist, I do much prefer cases that retain their original lines. Polishing may catch the eye with the shine of the surfaces, but is often at the expense of the sharp angles and/or contours that were originally present.
There really shouldn't be any rust present on vintage Ingenieurs, as the cases were made of stainless steel. If it is found, it may be around or on the movement, but that would indicate that significant moisture had permeated the case at some time, and would be a warning flag that one would be wise to heed.
Watches sent to IWC for full service may well return looking superficially "like new". But I stress "superficially", as a polished case, replacement dial, etc., will not look new to any seasoned collector, and the value of the watch on resale may deteriorate as well.
If you are not concerned about resale value, and expect to wear and enjoy the watch for many years, these decisions should be purely subjective.
Cheers,
Tony C.