It all depends how much effort you want to put in.
If you want to let software do it for you, ignore Photoshop filters and get a program like Dynamic Auto Painter. Photoshop filters alone are poor for emulating natural media.
If you want to diy, using cloning techniques, then Corel Painter and Photoshop offer good tools. In Photoshop, you should look at using the Art History Brush tool with good preset brushes that can be found on the internet. Or you can use the Mixer brush in cloning mode - again with custom brush presets - to make quite nice simulations. You can also use the Pattern Stamp Tool as a cloning tool for pencil sketches and watercolours, for example. But you need to Google on some tutorials - it is not a question of pressing a couple of Photoshop buttons to create a decent natural medias style. Topaz Simplify is a useful detail-reducing plugin - it is based on an excellent old plugin called "buZZ".
I do a lot of these and can say that Photoshop is a powerful tool for this kind of work and you don't have to buy Corel Painter.
If you want to paint from scratch, that is different matter all together but I don't think you are considering that.