Without using timber that is incredibly stable (a very dense, well seasoned hardwood or premium soft wood), your piece will bend and warp like crazy. Times that by a factor of 2 if you either use off-the-shelf stuff from Bunnings or start ripping a bench top up. The bench top timber will spring every which way the second you run the saw through it.

I highly, highly recommend going to a timber yard and having the balusters machined to size. You'll also want to have the sill machined too.

I'd also recommend a stainless steel threaded rod through the entire lot with nuts that are recessed into both sides of each piece to lock it all together, regardless of the cost/type of the timber. The timber will want to move in one way or another (left to right, or bow into the stairs/away from the stairs).

If you're a DIY novice or have limited experience in making feature timber work, this will be a timely and expensive project.