I used the hollow, square-profile vinyl parts from Lowes for an install in Waynesville 8 or so years ago. It went ok, but I was unhappy with the attachment to the slab at the foot of the stairs, as
@Jp8819 commented on above. It's been so long I don't remember for sure how I overcame the wobbles. I think I put a couple redheads into the face of the lowest step so the bottom bracket was held in 2 dimensions instead of just attached to the slab. The vinyl newel post fit over that, but I remember having to tweak it since it was designed to just attach to the slab. The owner ( retired guy in his late 70's)was satisfied and felt it was sturdy enough for him. I wasn't afraid it was going to fall over, but it wasn't rock-solid, either.
If I were to do the one in Basil's picture, I'd bore down through the slab and set a steel pipe "core" in new concrete 18" or more below slab, then figure out a way to slip the vinyl newel over that. Otherwise, the lower newel post is just a big lever to provide mechanical advantage to pry out the flange bolts!
I found this, not cheap, but I think the grandmaw don't want vinyl.
This one looks pretty, but I'd want to check how well it resisted lateral force if she had to lean on it suddenly. Looks like it bolts down with a flange and is bolted together at the joints to allow the angle to change.
A hammer drill and some galvanized or stainless threaded rod epoxied into the slab would be more stable than redheads or tapcons (concrete anchors).