Hello, Nick,
Welcome to the forum. There are so many "experienced" pilgrims here who can answer all your questions! They can also give you personal insights into various aspects of the Camino. I have only walked in summer so I cannot really help you in terms of weather, but I do suggest that you listen carefully to any/all warnings you receive regarding weather, especially in the mountains. There are a number of posts about weather, but the very recent one below this one "Walking through a blizzard in O Cebreiro" is very interesting. There is a link to their blog.
Take a browse around the list of topics and you will find a wealth of information on all things Camino! As far as a starting point, I came up from Madrid and began in Roncesvalles, the typical "starting point" from Spain. Many others begin in Saint Jean Pied de Port on the French side of the Pyrenees. Your starting point has more to do with time and personal choice, I think than anything else. Many would say, and I include myself among these, that your journey begins when you decide to make your Camino!
Buen Camino,