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.

I made minor repairs to mechanicals and replaced an aging garage roof but didn't really invest much else.

could it be the roof valley?

I would be getting an inspection so any roof or foundation issue would blow it out of the question.

If you primarily invest in WI and plan to continue as such, I would start researching boots-on-the-ground teams or experienced REI contractors that can provide you with a work of scope on the property - know what risk the property has physically before you invest.

Having boots on the ground makes the whole process even better.