How old is too old for LTR Duplex?

[Pepper Bradford]

I realize that this is both very subjective and varies by circumstance. I'm looking on the MS Gulf Coast in the Pascagoula area for 2,3,4-plexes as my first LTR investment. Because the shipyard nearby has been around since WWII there are a lot of duplexes that were built in the 40s and 50s. Right now I'm assembling a mailing list to start generating leads, and I'm wondering if I should automatically filter those out. As I understand it, anything built pre-Katrina that survived had a new roof put on about 20 years ago, but from Google street view it looks like a lot of them are metal. 

I guess the best question is this: Has anyone had experience with buying and LTR'ing (is that a verb?) a Duplex that's 60-80 years old?