There are options, optimals and what you have/can afford.
For mixing the crucial things are your hearing and to know the listening device you are using well, know as in what it sounds like with superb recordings and not so good stuff.
If you know the sound of your gear then you'll have a better chance of getting the sound you're after. Yes, the car, mono, monitors, ear buds, MP3 player, computer speakers etc. should be used to check the mix for translation issues.
I don't mix with head phones, I sue headphones when I mix to check for detail & for stereo image if I'm doing something that leans heavily into sound stage manipulation.
The Military Industrial Complex has an offshoot in music recording technology. Bigger, faster, more, new, constant "Growth" etc.
You can mix with an old stereo amp & speakers if you know them & that's all you have. The results will be as good as your ears and awareness.
I went from an old stereo to 5" studio monitors and then to 7" Adams. Improvement came as I went along due to awareness, learning & gear.
There was also a dip in quality with each change as I learnt the new sound in the room, relative to old mixes and reference tracks.
I listen for pleasure with Sennheiser HD 515, (open) or ear buds, (closed) if out walking, (I did use headphones outside but they don't fit well under my hat & the back of neck band ones aren't too flash). When recording I track with MOREME's, (closed), as they are closed and accentuate mids so that following the backing is easier. I have a couple of old set of phones for mixing detail - Yamaha, (closed), Sony, (open) etc.
It's more about your hearing and your awareness than a piece of kit.

Cheers
rayc
"What's so funny about peace, love & understanding?" - N.Lowe