My Opinion, Take off all the old roofing right down to the roof boards put PLYWOOD over the roof boards not Osb. Osb is not recommended by steel roofing manufacturers because the screws wont hold. Also, there is a different shorter fatter screw to use with steel over plywood The long skinny ones are for 2x4 purlins. If your house has big wide mouldings,at the edge of the roof, get rid of it. When you get to the peak make sure you put something between the roofing and the cap to keep the snow and rain from blowing in. The best most expensive steel roofing is standing seam. Regular steel roofing made these days is nowhere near as good as it was in 1955. A couple of years ago I removed steel roofing off a porch on my house to change two roofs to the same pitch. On the back side it was stamped GUARANTEED UNTIL 1928. One section of my house still has that same roofing on it and there are no plans to change it. Read what the manufacturers say. Manufacturers have several different mouldings to fit most situations.