Nearly all DVDs (of theatrical movies) are progressive.
29.97 fps video (on DVD) are usually from a video source (as opposed to film).
Since the NTSC standard uses 29.97 (interlaced), progressive 24 frame film material is slowed to 23.976 and a pulldown flag is added to tell the hardware/software to convert (telecine) on-the-fly to make it 29.97 by duplicating and interlacing frames (the pattern is unimportant at this point).
This is why there are Progressive Scan DVD players. They don't convert the data to 29.97, they playback the 24 frame progressive material as recorded.
So although (say the Star Wars) DVD is telecined on playback the actually data on disc is really 23.976 progressive. When re-encoding the flags are ignored and the original 23.976 frames are re-encoded and new pulldown flags are added at the end so it'll playback correctly.
Occasionally you will find true video sources (which is completely interlaced with no duplicate frames at 29.97 fps), and then you have to mess with the deinterlacing (or leave it be, depending on what you want to do with it).
You may also find film sources that have been telecined and recorded that way (such as on laserdiscs or VHS tapes which don't allow for progressive material). Then you must inverse telecining (IVTC) because the material is recorded interlaced at 29.97, not converted to it during playback.
So yes, the material started progressive and ended progressive.
This is one reason some of us can't wait to play with HD content. They allows for true 24 frames. There is no NTSC and no PAL. Hopefully interlacing and telecining will soon be a thing of the past.
HD-DVD and Blu-Ray discs don't even allow for 4x3 content, meaning non-anamorphic discs can't exist.
As far as stipping material, if you are converting down to a single layer DVD (DVD-5), absolutely strip extras, languages, and subtitles that are of no interest to you. I personally would probably just use the Movie Only Mode (since you probably don't care if your extras are anamorphic).
The more space you free up that way, the higher the encoding quality will be on the movie. This is true regardless of compression (transcoding) or re-encoding.
(This is also redundant: Methods of re-encoding like DVD Rebuilder provide higher quality than software like DVD Shrink. The reason is the video is fully re-encoded (which is slower but more accurate). DVD Shrink is a variation on software used in digital broadcasting: "they do not have to decode and re-encode the entire video stream, but only part of it". DVD Shrink probably produces invisible results to 90% of the people to about 80% compression. After that they develop artifacts much faster than re-encoding the video at a lower bitrate which in some cases can almost be halved and still produce a nice picture.)
As far as the ITU aspect ratio, don't worry, it's just plain complicated to explain. The avisynth filters can let you add noise reduction or sharpening, etc. if you are a hopeless tweaker. Again, not particularly necessary.