Honestly and from experience, I refuse to work with anyone else models other than my own - unless it's only being used for distant context. In my workflow, architects models are used only for previz and that's only if I don't have time to remodel it from scratch first.
Of course the time taken to remodel a building from scratch is factored into the initial quote.
Far too many times, architects say "we already have a model" in the hopes of getting the cost down, and it ends up taking you longer to fix their model, than it would have taken to just model it from scratch.
And you can never really foresee problems with architects models, things may look fine on the surface, but then when you get into the details they become a nightmare to work with, but by that point it's too late to go back to the client and say "actually we need to model it from scratch, that'll be an extra £XXXX". There are just too many possible problems, smoothing issues, displacement not working, holes in the mesh, items with different materials attached together, simple innacurracies, missing details, geometry clashes, coplanar faces, single faced glass etc etc etc, and some of these your don't realise until you come to render it.
So to answer the question, I simply do just model everything from scratch, to save a lot of headaches further down the line. Although it can feel a bit mundane, it can save you a tonne of time elsewhere, things like materials and scene management/organization etc.
Paying me to model the buildings from scratch isn't optional on a quote. If they insist on me using their model, then I insist on the cost going up for 'troubleshooting and cleanup'. And if you're being paid for it and you've priced for it properly, then the time taken to do it shouldn't be a problem.
There's also the added benefit of getting a much better understanding of the building and the relevant spaces when you model it from scratch.
I find SketchUp is the worst, Revit is ok - both are heavily dependant on whoever built them, and anything that you can't import natively i.e. Vectorworks isn't even worth considering.
So always cost and schedule time for modeling from scratch, and if it transpires that the architects model is excellent, problem free and usable, then win win, you just made some extra profit and got yourself a more relaxed schedule.