The only "basic" in the list you wrote would be the transmission cooler, if you just created your own "towing package". Sure, ideally a higher output alternator and stuff like that would be nice, but to give you an idea:
On my 98 Tacoma, there was no tow package. Just your typical cold weather pickup (meaning it does have a more powerful starter and maybe a couple other things). It could tow 3500 as-is, from the bumper. With a hitch it's 5,000.
I added a brake controller, hitch and air bags in the rear suspension. The air bags make a
huge difference but aren't necessary. Mine is a 5 speed but had it been an auto, I would have needed to install a transmission cooler.
I pull a 17'
Bigfoot which is likely between 3,000-4,000 pounds, and doesn't exactly have an aerodynamic profile. I don't know what the tow capacity is for a late model 4Runner but it at least has a much more powerful engine than my 3.4L V6. My truck pulls (and stops) my
Bigfoot just fine.