You do not need to think no tenant, but rent goes down 25%. as long as you lower rent to certain level, there are must tenants.
The local economy is important. Usually if there are only one major economy (like oil), it is high risk.
Reserve, I think 6 month all real estate PITIA is enough.
And you can use credit limit (credit card or line of credit as reserve).
Just my 2 cents.