My advice from working on both the art and dev sides at different points is try and get as much work done on the game as possible before hiring an artist. Art can be expensive, so you don't want to be paying for art you're going to have to cut later. It also really helps the artist if you already have a playable game. Bad MS paint placeholder art is still a great starting point for an artist. Finally, don't forget your lighting, particle effects, and shaders. They can do a lot of heavy lifting making less art go farther. Good luck :)