You mentioned that you'd have to paint. No so with the Koma or Azek. They come white.

As for poplar, like any other wood, you want to keep the column wraps about 1/2" of the floor to prevent rotting from water. Cover that gap with trim.

Take a look at vinyl beadboard for your soffits and overhang. You'll be glad you did years from now when you don't have to paint it.

I've blended M/Free products, both vinyl and aluminum, and they actually work well together.

Good luck on your project.

The Porch Guy

But as far as long-term longevity outside? I'd never advocate poplar.

Mongo

Exterior pop;ar will experience wide changes in humidity, leading to any finish cracking and allowing moisture to enter.

Cypress is actually much better for outside use, but you need to find someone to make the moldings you want.

Or use a synthetic.