Making sure the heating works, the roof does not leak the place can be made secure (doors and windows) are all going to fall on the landlord.

Windows are my biggest issue on rehabs good luck "replacing" them.

The house has a finished roof, exterior windows and all of the weather wrap, but no siding and no interior pictures (assuming it would be at the most, dry-walled inside). 

I have several houses in Orange County, CA which I would like to replace the windows.

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How are the windows and what are they made of?

For me a sign in the window of the property and word of mouth advertising by friends, family, neighbors and of course yourself seem to provide the results desired.

I told my wife that I would not throw $20-30k out the window in taxes and penalties to buy the property.

I've had some success in those windows....

Brtan, I believe the value of the 50% rule is in the idea that expenses will be 50% of rent so when PI exceeds that the investor should look closely at what will be coming up, i.e. new roof, furnace, siding, windows etc.