A back corner of the garage is rotted, there's a hole in the roof, studs are visible on the walls and it needs a total rebuild.Who do I contact first to begin the process?

Two years of that lost income would pay for a new furnace. 3-4 years would pay for a new roof.

Yes i know it has property taxes that eat into profit margins, lets forget about that aspect though. 

Still need to budget for roof, appliances, hot water heater, hvac, etc.

Once it is renovated it will be put out for tenants, Currently in the process in replacing the roof and redoing the driveway so have about 6-8 weeks of work depending on how fast people work and how cold it gets up in MIOnce that is done then it will be switched into an investment property

Like how can I judge if the roof looks good or anything else that may come at a hefty price to maintain or fix in the short term?

The home will be empty for around 3 months while we redo the roof etc so would it make more sense to get vacant home insurance initially and then switch it to another policy that will have at least 2 mil in liabilityThank you (I have no agent yet so if you take the time to reply with info and are licensed a deal is yours)

Mine has roof, floors, hvac, appliances, hot water heater, bath remodel, kitchen remodel, exterior paint or siding, etc.

I hit plumbing, electrical where necessary, foundation, roofing and HVAC.

If roofers are making noise there is nothing you can do it is expected but if contractors are dropping hammers off the roof you would be taking action.