Your complex evil characters might want to avenge, well, nearly anything. And even if you used that as their motivation before, it doesn't mean you can't use it again. It can still work.
Don't pay it all up front, and don't agree to do so. If you are absolutely, flat-out broke, you should still be able to pay something, even if the artist hand waves and doesn't want anything for their work. Be good to your conscience and at least ask if you can make a small donation to one of their three favorite charities.
If the covers in your genre's section of the bookstore are all orange, should your cover be orange, too? It's hard to say. You want it to look like it belongs in that section, right? But you also want it to stand out. I would say, if you are a new author and you are predominantly selling online, you need to consider how your work is going to look when it's shown with others in the genre.
If your southern American characters sound like Gomer Pyle, and your Mexican characters sound like Señor Wences, you are probably not doing such a hot job with depicting their accents. Same with a British character who ends up sounding like Dick Van Dyke in Mary Poppins. Just, don't.
The biggest and most measurable benefit is that it keeps you writing. You can often spark creativity by simply being creative, that is, you write five or seven days per week, and you can fill up that writing time fairly readily. But if you only write three times per month, you may find you have writers' block when you make the infrequent attempt. There is something about the pressure of deadlines or at least the pressure of your own internal expectations. It helps to not have a blank page to stare at all the time.
When Jewish characters (for example) are on the screen, does the audience get more than an occasion reference to Chanukah? Or do they just get a surname, or a trope? Or worse, do they get thinly-veiled anti-Semitic caricatures? Are LGBTQ characters defined by their sexuality, or are they stereotyped, or is it no big deal? Or are they killed off quickly, once they're no longer useful to the plot, the show runners, or the network?