You have a complicated path here. Not only do you need to convert from PM to ID, you have to FIRST convert the older PM5 files up to at least PM6.5 so that InDesign CS6 can handle them (CS6 being the last version that could do this conversion and which is not available anyway, so you have another problem facing you).
Still, even if you can get a script for this, the results will not be what you expect. And "quickly" isn't going to be a thing.
That being said, It will take far less time to massage these files after conversion than having to rebuild things from scratch, so I guess it's up to your Quality Department to think strongly about what they want to gain out of this, and are these files worth the time involved, especially if you need to pay someone to do it.
There needs to be a lot happening for a successful/reasonable conversion.
A) You need all the links (pictures and graphics) supplied as separate files. Many people "back in the day" unknowingly embedded these files (as that's how Pagemaker worked originally, until later versions were able to simply "Link" those files), but. InDesign won't have a good time with those. And even if they WERE linked, but are now unavailable, you're toast anyway. Many of those linked files may need to be upgraded as well to work in the ID world.
B) There will be font issues. There will be quite some time locating matching typefaces and some simply may not be available anymore for current systems or be an exact match for old fonts.
C) Layouts will change and need some (or a LOT of) reworking to get back in line, as the way these programs work under the hood are very different. If the file is simple, this may not take much time at all, but if you have a larger complicated file it will take some time. Even converting from PM5 to PM7(or PM6.5) saw text reflow because the methods for text composition got better as PM developed, so things like spacing and kerning changed.
A few months ago, a client secured me to convert a batch of several legacy files from the '90s. These were published books that were needed again for reprinting these many years later, none of which had PDFs (since that wasn't a print workflow in those days), and many of which they didn't even have hard copies of anymore. It took many hours, but it was done! (To be honest, I kinda enjoyed the challenge of it)
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