So, I haven’t seen this topic yet, but as the title suggests, this is what I’m looking to export my models into: The Source engine, and specifically, Source Filmmaker. I’m looking to make my own models for animations.
Welp, onto answering the export checklist questions! My answers are:
Where do I want to export to?
Source Engine/Source Filmmaker
What axis is up? (“Handedness”?)
Z (right-handed).
Does the scale set in Blender come through?
Yes, but from what I understand and have read you can change the scale multiplier in the QC file after exporting, and thus change the scale of the model (see the link).
https://developer.valvesoftware.com/wiki/Blender_Modelling_Walkthrough#Compiling_the_QC_file
Does the model have an appropriate origin?
I’d just create the model from Blender’s origin for simplicity’s sake and peace of mind.
I’m not doing the last two questions because obviously those you ask yourself when exporting. 
So, this has enlightened me a little more in respect to SFM, and I hope it helps anyone else looking to do the same!