You can modify a preset in visual ways only. more or less. (scars, holes, coloured effects, water inside, fire, flashing animations ect) I've added underwater effects and ended up with them casting a shadow but they often break, so don't do that.
They're visible to people who have the exported file, so you, anyone you give it to, and anyone who downloads it if you post it online.
My tail works normally with them. I'm not sure why anyone would say otherwise.
Price generally depends on the person making it and how detailed/complicated the preset is. Are you looking to get a preset?