"I don't think it would take much to get plants growing in Hellas. The dust storms could be a problem, but if you could keep the basin wet I bet the dust would stick to the ground.

I wonder if there are any plants that would grow there now without too much modification?

Now the rest of the planet, that is another challenge. "

Go to any mountain which is 20,000' or higher- most are over in Asia. At 20,000' it will be about 60F cooler than compared to sea level temperature, so if it's 89 F in daytime at 1000' elevation near mountain base, at 20,000' it will be 32F [freezing]. So, in the winter, or at nite in the summer and it's at 69 F at 1000', it will be 20 F below freezing at 20000' level.
So when you are at 20,000' every nite it is going go below freezing, and on warm days it might get above freezing. Now, if you measuring the ground temperature and if you measuring bare rock or dry dirt and it's in the sunlight, and it's not very windy, the rocks or dirt may be 70-80 F or warmer.

Anywhere on earth at 20,000' is going to be above the treeline- meaning tree don't grow anywhere near this elevation, anywhere on earth. Though other plants can grow above the treeline. But to get conditions closer to Mars you need Mountains on Earth which at least 50,000'.

Suppose we had huge mountain rising out the Pacific- which dwarfed Hawaii- and the Mountain had a high plateau which was 50,000' above sea level- that plateau would be warmer and milder and generally more friendly to life than Hellas on Mars- during day and nite it would much warmer than anywhere on Mars. And with similar training that makes an Olympic champion weight lifter, one might even get to point of being able to breathe from a oxygen mask instead using a pressure suit. Though one probably couldn't breathe for hours [or sleep]- maybe 10 mins or an hour or so.
At this 50,000' dry rocks and dirt could get +80 F and if threw water on them they cause the water to boil.
There probably wouldn't be much water, snow or ice on this Plateau- it might even be drier than Mars.