Hardi Plank, by James Hardi is a good product, but their version of roofing had to be pulled off homes as insurance would not cover the product. Do not know why.
I use hardi planks on many jobs where the wood is touching the ground or concrete and they, the customers are tired of rotting out areas. Hardi can lay in the ground and will not do any of the things that lumber will do.
Hardi comes primed only, you do need to paint it with a finish coat. It takes paint quite well and has that nice grain look.
Houston, my old home, has that humidity and this would be ideal for a siding. I do remember with all that humidity, you do have algea, mold, and stains from that humidity that will need to be addressed occasionally. A mild power washer will take care of that.
Your question on painting, there is home center paint and there is paint dealer paint, so what I am saying is that a quality paint will last much longer than a non quality paint. I would say that a quality paint will need a new coat mainly due to fading and loss of luster about every 7 to 10 years depending on the conditions of the sun and other natural factors.