What should I charge for 2 months of freelance work?

I have an opportunity to build a simple mobile app using Unity or Unreal (my choice) where the user can navigate a virtual space based on a real building. The company doesn't have models for the virtual replication of the real environment so I either have to model it myself, outsource, or use similar assets that I find online. They want this done in 2 months and the user needs to be able to navigate 10 rooms in this building with text pop ups on what each room is used for. How much should I charge this company for this kind of work? It seems simple to build/ code. The only annoyance is that I'm not a modeler so i would need to figure out how to model the environment as close to what they want as possible.