Ask HN: What questions to ask to determine the right 3D application framework?

A week from now I have a call with a structural engineering company that wants a 3D Application. The account manager has given me no information and seems OK with me going into the call blindly.

Having never designed a 3D application, what are some good questions to ask so that I can choose the right framework ? I currently work in .NET and C#, so Unity is a good choice for me. But I want to choose what is best for the customer not me personally. Given that this software is for structural engineers (e.g., it doesn't have to be pretty) and a user base of < 1000, Unity seems like overkill. Any thoughts ?