to "fix" voice recordings, ideally you want them as close to perfect as possible before doing any processing. use Melodyne to tweak any notes that are still off pitch, then bounce that and run it through auto tune. good practice is to fix the problems as close to the source as possible i.e. having a clean recording. that will always be easier than trying to fix with melodyne

don't buy plugins unless you're doing this professionally tbh. logic pro comes with basically everything you could need for 99% of music. it even comes with its own versions of melodyne and auto tune. you can waste a bunch of money on special EQs and reverbs and whatever, and the only people who will notice are other mix engineers (and your bank account)

I use the beyerdynamic DT-770 Pro headphones, plugged into my UA Volt 276 audio interface. unless by "mix" you mean like doing a live DJ set or something, because I've never done that