Does "Un calido saludo" roughly translate as a "warm/friendly hello?"
How could I send a "warm and friendly hello" in Spanish, avoiding any weird hot/sexy connotations? Thank you!
That sounds good and does not have any weird/hot/sexy connotations. You can also say "un cordial saludo". By the way, don't forget the tilde on "cálido".
Cálido means warm to the touch but I've never heard it used metaphorically to mean kind/friendly so I would avoid saying un saludo cálido. Un saludo cordial is much more natural and will not be misunderstood, but do remember to put the adjective (cordial) after the word it modifies (saludo). If you would like to close a letter with friendly note you can sign "con cariño" which translates roughly as "with fondness".
Mil gracias, Guillermo!