Hi! I've worked on independent animated series before, I think I can guide you a little.
First, I think the price you want to pay is too low, considering that creating and animating 5 characters for 15 minutes is a lot of work. I would say the price should be at least $1000 for a decent, independent work (a bigger studio should be even more expensive).
For the timeline, If you already have the story, you should start by creating the characters, scenarios, maybe some concept art, then a storyboard, and finally you get to animating. The voiceover can be made before the animation, or you can get the animation dubbed later.
Hope we get to work together.!