Question-and-Answer Resource for the Building Energy Modeling Community
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If you are new to OpenStudio, I would highly recommend starting with the introductory tutorial and model creation guide. For creating and editing the geometry of the model, you are correct that you can use either the SketchUp extension (better for more complex geometry) or the FloorspaceJS tool within the OpenStudio Application. Note that if you are using the OpenStudio Application version 1.0 or higher, you will need to pay for SketchUp Pro 2019 or later.
If you have current data on energy consumption and installed equipment, then you are likely going to create a calibrated energy model -- one where the model's predicted energy consumption matches the actual building's energy consumption within an acceptable range.
It is very hard to answer your three questions without knowing more details about the buildings and the efficiency measures you want to analyze. OpenStudio should have all of the features you need to:
After working on your project some more and developing specific questions, I would recommend searching Unmet Hours to see if others have already posted the same questions you have (or at least something very similar to it). The chances are very high, as over 3,000 questions to date include the openstudio tag.
