I have the excellent Total Commander; it has a built-in DIFF tool that's good enough for most of my needs nowadays. Back when I needed something significantly more I used
ExamDiff Pro, which is very good. You can trigger it from Total Commander.
I bought my Total Commander license in 1999, I think, and have never been asked to pay for an update. I also have XYplorer, which is not only portable, but offers a definite lifetime license and is updated so often you feel surprised if you check and find you're already up to date. It's a second string for me, but has done some useful work. Thirdly I have
ZTreeWin, but for all its virtues, it's more a nostalgia trip now
If by
index you mean something like a catalogue, I'd suggest
Cathy, which is small, lightning fast and free. It can search duplicates across catalogues, so should be able to meet your offline requirements (I think). I don't find the UI as intuitive as all that. There's also a command-line version for updating catalogues, good to put in your scheduler.