Yeah, it can certainly be a daunting task. MC does make the task easier once you understand how all of its tools work and a few tricks on how to use them in sometimes unconventional ways to get the results you're after. If you haven't already, experiment with the Library Tools -> Fill Properties From Filename... tool. It might help ease the pain of filling in missing metadata, which I'd honestly make the first step in your cleanup/de-dupe process. Clean up the basic metadata, (artist, album, date, track #, and name) and then you can easily find duplicates just by setting up a view similar to what I described above. Once you've de-duped everything, then you can focus on tagging files more thoroughly than the basics if you want.
I really like using the calculated field to define the criteria that qualify files as "duplicates", because it allows me to use that as the Group By field in the view and group all of the duplicates together. Then you can sort inside the groups by whatever criteria define one to be better than the other so the best one bubbles up to the top and at that point it becomes trivial to quickly scan the entire the list and confirm the one you want to keep is the top one, highlight the rest and delete them. If there are files you want to delete that have metadata you want to keep in the better copy, just Copy the file with the metadata and use Ctrl-Shift-V to get a list of tags to paste into the new file. To make it even easier to mass delete the duplicates once you're done examining the list in detail view and copy/pasting any tags you want, switch to thumbnail view with small enough thumbnails that you get multiple columns. Now the file on top (the chosen one) is in the leftmost column meaning you can easily highlight everything else to the right of that column and delete all of the duplicates in one fell swoop. This setup works so well for me I was able to visually scan and eliminate 80k+ duplicate images out of my photo collection in just a couple of evenings of working on it.
It's certainly pretty powerful software. I'm constantly amazed at the things I can coerce it into doing.
But to quickly answer your other question about filtering on missing metadata, you can use the Search Box (or setup a Smartlist or View) using a query like [Artist]=[] and that will show you all files that have an empty Artist field. You can do the same with any field you consider missing. Album, Name, etc. and wrap them in () to "OR" them. So ([Artist]=[] [Album]=[] [Name]=[]) would do a search for any file that has either an empty Artist, Album, OR Name field. Check out the wiki for more details on how the search stuff works. It's pretty powerful, but the basics are easy enough to learn. You can use the Search Wizard too if you prefer a GUI.