Speaking of budget, the hiring of an illustrator is one of the greatest expenses I incurred while publishing. However, they are artists too, and deserve to be paid as such. If you have a finite budget, you can still have wonderful illustrations, just as if you had oodles of money to spend. Be realistic about your budget for that illustrator. I wanted this one illustrator, I won't name names, but her reality was 400 a picture, my reality was, well let's just say I live and write on a teacher's salary. Oh what I wouldn't have given for her work...again, reality.
Once you find that illustrator that trips the light fantastic for your words, let them create the final dream. Oh yes, you have your ideas to shoot at them, but if you can, let them have as much vision as you can muster...worst case, you sell 1000 books...best case...you are now a star and are appearing on the Today show on their author segment.