I like a lot the way you prepared your question. Congratulations.

1) You are confusing things.

Your scanner can scan at 2400 ppi, this is 8 times more than the theoretical 300ppi, and it would be used to scan, for example, a postal stamp at a high resolution to make it bigger.

2) Forget for the moment your unit conversions "nd 300 PPI scanned at 100% scale equates to 300 DPI" Let us use just PPI. DPI is a unit used on printers, but let us use it for now interchangeably.

3) I understand that the illustrator is making the images almost at real size (in this case scanning at 300 ppi is ok). For archive purposes, and to leave some room to the design to accommodate images, and play with the design, I recommend that you scan them at 600 ppi.

4) I am not sure if a scanner is the best option to scan a watercolor in a very texturized paper.

The light is shooted at a very specific angle and could or could not be the best lighting to enhance the texture of the paper.

Do you want the texture? Do you want to flatten the texture to show only the color? Do you want to show more texture?

Probably a better approach would be taking a photograph. With that budget you can afford a good quality camera, lens and illumination.

I like a lot the way you prepared your question. Congratulations.

1) You are confusing things.

Your scanner can scan at 2400 ppi (forget the 4800 for now), this is 8 times more than the theoretical 300ppi you need, and it would be used to scan, for example, a postal stamp at a high resolution to make it bigger.

2) Forget for the moment your unit conversions "300 PPI scanned at 100% scale equates to 300 DPI" Let us use just PPI. DPI is a unit used on printers, but let us use it for now interchangeably.

3) I understand that the illustrator is making the images almost at real size (in this case scanning at 300 ppi is ok). For archive purposes, and to leave some room to the design to accommodate images, and play with the design, I recommend that you scan them at 600 ppi.

4) I am not sure if a scanner is the best option to scan a watercolor in a very texturized paper.

The light is shooted at a very specific angle and could or could not be the best lighting to enhance the texture of the paper.

Do you want the texture? Do you want to flatten the texture to show only the color? Do you want to show more texture?

Probably a better approach would be taking a photograph. With that budget you can afford a good quality camera, lens and illumination.