When building a model you want to get your sources to be as close to the real thing as possible. Just like a game of
Chinese Whispers, every reproduction introduces some error, and depending on how much time someone spent on trying to make something accurate, the amount of error varies. For best results you want your bus to be an exact copy of the real thing, but obviously it will never be perfect and one must reach a compromise between perfection and finishing the project.
The best source of reference is the real thing, or the diagrams that were sent to the manufacturing teams to tell them what to make. The latter is difficult to come by, and usually only for old vehicles where the blueprints are in a historical archive somewhere. A current manufacturer isn't going to give you that which tells you how to copy their bus in case you sell it to a rival manufacturer. Taking a tape measure (or other measuring equipment) to a real bus will give the best results, but you'd probably need to arrange a depot visit to do that, and they're obviously not going to let in a kid with no previous experience. SCS was able to 3D scan the new Scania to make their model, but obviously they're a company with a lot of existing models to lean on.
Service and Operators' manuals can also be useful, but remember that they're written before the first bus is produced, so they have to be based on pre-production vehicles which may differ from the final product. They also have revisions over time if things are improved on the vehicle.
Well-lit, in-focus and high-definition photographs of the real thing from every angle you can get are vital, but I wouldn't trace from them. The lens of a camera distorts the image (with more distortion the further from the centre you get) and so they don't actually tell you the truth.
Paintings, sketches, paper bus templates, brochure artwork, "blueprints" from blueprint sites, etc. are all artist's impressions of the thing rather than highly accurate reference and should be avoided. They may look recognisable, but when compared carefully to the real thing or to accurate diagrams they quickly show up their issues.