No I do think a brushed waterfall is fine in TF2, Thats what I did in my tfbridge map, and it turned out all sexy like. All it took was putting the right water texture on of the sides of the brush with everything else nodrawed. Then I made a new brush and used a scrolling water texture and placed multiple burshes/sizes of the scrolling water a few units in front of the water texture. (the water brushes had to be the same height as the waterfall to make it make sense.) And since my waterfall was very large, I added white env_smoke all along the bottem and top to give the illusion that mist was forming.
I would say download my map and take a look if you want an example on how it's done.
Lastly if your new to mapping, I think editing a Valve map is fine as long as you get off that when you have a handle of things and start making your own maps. Custom versions of Valve maps is kinda...meh...around here