In my experience through a dozen plus years on the topic, these issues are best discussed and defined at the outset whether you're outsourcing the work or doing it yourself.
1. What is the initial use of the model?
Start with a MVM (Minimal Viable Model ), volumetrically correct and add detail from there. Interior designers will need different detail than someone implementing CAFM tools, then will an architect adding an addition or an adaptive reuse project. Custom windows, Wall Types, Beams, Fire Protection, Molding, etc. can all be added later so don’t pay for what you don’t need.
2. What technology is being deployed?
There are a variety of technologies out there, what are you or your vendor using? How much experience do they have. Do not be afraid to ask for or create a technology road map and ask for demonstrations, experience, and documentation of previous projects. Technology and data capture can be and should be quite different for a 1000 rooms in a hotel vs. a theatre with non-orthogonal walls, balconies, and lots of trim detail. Will it be HDLS (HIgh Def laser Scanning), PPLT (Point to Point Laser Technology or P2P), Photogrammetry, Hand measuring and graph paper? There are price considerations to be had with each.
3. Architectural Intent?
Hard to believe this makes into the top 3, however, you need to discuss how you want to deal with non-conforming issues. For example, less than a .3 degree deflection of a wall in plan view will cause +6” over 100 feet. Who cares? Do you? It’s important. As non-ortho connections in Revit cause model errors, something in new construction Revit wants you to avoid, however when modeling existing not every building was built perfectly square. Would you rather see the building modeled with regards to architectural intent and have any non conforming issues annotated or do you want it modeled as it exists.
There can be lot of confusion in the marketplace with regards to laser scanning, documenting and modeling existing buildings as there are no standards in place, nor certifying boards for anyone doing this work so until that day happens the onus is on the consumer to ask the questions and help manage the process to get the model they need.