Whether ID can open your PM files successfully is something that you will find out on a case by case basis. The results will be anywhere from near perfect to unusable.
InDesign does not depend on any external applications for the creation of PDF.
Why don’t you download the demo and try it out?
In terms of converting existing PageMaker documents to InDesign documents, although InDesign can do such conversions, depending upon how you created your PageMaker documents, you may require anywhere from minor to major manual adjustments of the results. For example, tables in InDesign are very different from those of PageMaker. The layout engine is dramatically different (we would like to believe that the results are also dramatically better), but different means that you really need to look at the results and may need to tweek your layout in InDesign. The difficulty and magnitude of such required tweeking obviously depends upon how complex your layouts were in PageMaker and the degree to which the InDesign layout must match that of what you had in PageMaker. Use of non-Western Latin fonts in PageMaker may make the conversion more difficult. Any use of Windows OLE (object linking and embedding) where your objects in PageMaker reference objects within other Windows programs will not carry over directly to InDesign and you will need to manually handle such situations if you have such. The PageMaker to InDesign conversion is a one time investment per document, but could be time consuming and frustrating.
In terms of PDF production, the built-in PDF export capability of InDesign is in our belief the best in the industry. There is no intermediate PostScript involved and the resultant PDF completely supports the entire imaging model of InDesign in particular and Adobe in general including live transparency and color management.
I think the real issue comes down to what the future is of the PageMaker documents you have. If this is a last time creation of PDF exercise, then you might be better off maintaining a copy of PageMaker 7 on an old Windows XP system and do the work there. If you need to be able to do anything with the PageMaker documents in the future, including passing them off to someone else for any future updates or publication, the conversion to InDesign is almost a necessity.
I am sure other users will offer their opinions here ...