No idea how old the roof is.

For example, do you recommend always bringing in an HVAC specialist, roofing specialist, plumbing specialist, electrical specialist, etc just to make sure that part of the potential rental is working optimally.

Still have to budget for roof, appliances, hot water heater, hvac, bath and kitchen remodel,etc.

So if you eliminate all of those properties that have old roofs, mold, settlement issues, outstanding code violations, and more of the why step in that - I'll call those "bad deals," you can get on with analyzing deals that are worth your time and possibly worth doing.  

The unit is 3 stories tall with a mansard roof, so the work will be up on scaffolds. 

Without seeing the property, I would assume it would need new electrical, new HVAC, plumbing, drywall, finishes, appliances, and you would want to talk to an expert in restoring properties due to fire---as their could be issues with the roof, siding, windows.

Fix important stuff like the roof, siding, doors and windows.

Obviously, there are minimums since the cost of a roof or rewire doesn’t change with the ARV.

I only know the big companies and they're quoting a lot higher than I usually pay here for similar roof. 

The seller agreed as long as we agreed to the asking price, there was a patch needed in the roof and a door had to be replaced.