I'll go out on a limb stating that the Canon 85mm ... (
Yeah, the Canon EF 85mm f/1.4L IS USM is a wonderful lens.... But it's also a $1500 lens. Not sure why you're singing the praises of a lens that's triple the budget the OP mentions.
More realistic for the OP, with their $500 budget....
Canon EF 85mm f/1.8 USM is on sale for $299 right now (lens hood sold separately, recommended).
Canon EF 100mm f/2 USM (not the macro lenses) has been quietly discontinued and is no longer available new, but can be found used for under $400.
Regarding what exposure mode to use... It depends upon the situation.
With very stable lighting conditions, I prefer to lock in my settings with fully manual exposure. If shooting indoors that's usually doable for an extended time. But if outdoors, one should check every 15 minutes or half hour to see if lighting has changed enough to require tweaking the exposure settings.
With variable lighting conditions.... which include following subjects in and out of shadows or through brighter and more dimly lit areas of an indoor venue... one of the auto exposure modes may be necessary. More and more I use M with Auto ISO (which is not
manual mode). Other times I use Aperture Priority AE or Shutter Priority AE. I very rarely use Program AE. I never
use Auto ISO in conjunction with Aperture Priority, Shutter Priority or Program AE. And I never
use full "Auto" or any of the Scene modes such as "Running Man/Sports". Those dictate far more than just the exposure settings. They take over control of AF, file type, and more. No thanks! (Heck, my primary sports cameras now don't even have Scene modes... They do have a "Point n Shoot" full Auto setting, which I avoid like the plague.)
You should learn how to use the AE modes too. M simply isn't the answer for every situation. There are times when AV/A, Tv/S, M+Auto ISO, or even P can work better.
Whether or not M+Auto ISO AE is usable on a camera depends upon how it's implemented. Older camera models I used had Auto ISO, but it didn't have a means of setting an upper limit and couldn't be used with Exposure Compensation. As a result, it was largely unusable. My current cameras allow both these necessities with M+Auto ISO AE, so I use it now fairly often.