Flat Rubber Roof Replacement questions

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We have a building that has a pretty rough roof, One section has been patched and I'm thinking we need to just replace the thing. Will be getting pricing from vendors but thought id check with the community about any good thoughts or rough price guides. 

I believe its going to be about 1500 sq ft (currently a 2 layer in place)

Suggestion for basic 10 year warranty roof is 45 mil EPDM Rubber Membrane. 

Let me know thoughts regarding flat roofs as this will be my first replacement if we do it.