1. The best is to get blueprints. Regarding the photos, they should be as much accurate as possible and from different angles. The more reference images, the better.
2. Quality criteria depends on the place where the model will be used. There are some general criteria such as construction, textures, rendering settings, cleanliness, mostly quads.
3. The price depends on the artists. The better portfolio and skills he has, the bigger price will be. Furthermore, most designers evaluate their work by hours / days the project will take them, so it depends how big your project is and how fast you want to have the final result.
Good luck!