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I would suggest playing around with the artistic filters in PS, adjusting the sliders to taste, and/or trying one of the inexpensive plug-ins from Topaz Labs before buying an expensive "Paint" program----unless this is something you really really really want to get into.
Once you have an image file post-processed the way you like, the selection of media to print on will help the image take on the watercolor or other artistic slant characteristics you're after. Fine art matte papers, texture, bamboo, sugar cane, rice paper, canvas, etc.
I think the best thing for post-processing is to not overdo it-----unless that's the flavor of the day.