Metal Roof Over Shingled Cold Pole Barn

Hello all. I have a 26w x 30l second garage that a plan to put a metal roof on. It's about 30 years old and has what I believe to be the original asphalt shingled roof. The shingles are showing their age and some of the nails have come loose. The garage is uninsulated, unheated, and drafty. it's basically just cold storage. That existing roof contains ridge vents although I wouldn't think they are required.
My plan in to cover the existing with 26 gauge (most likely) standing seam metal. From what I have read the correct approach, assume no soft spots or rot, would be to install horizontal purlins on 24" OC and verticals on the ends then cover with the steel. I plan to remove the ridge vent and cover it with a metal ridge cap. The structure currently has gutters and I was going to overhand the metal an inch.
Am I on the right path? And what size should I go with for purlins? 1x4 or 2x4? Any suggestions are appreciated.

Thanks

Installation details would mostly be dictated by the panel that you choose to maintain warranty enforcement.

That said, you are on the right path and have a solid layout.