You shouldn't be too concerned about existing industries being changed during exposition, so picking a settlement based on primary industries is a safe bet. People cannot alter what already exists during expo unless they own/have permission and manually remove what already exists, and then add in something else. It would be much more cost effective to expand than to destroy the current industries.
You don't Have to farm and gather your own raw materials, but it may help to do so to some degree. Elyria is meant to have a functioning economy where people can provide raw materials for others who process them and sell them to the eventual consumer. However if you don't want to pay someone for your silk and set up your own farm, you will spend more time working but have less costs with regard to materials. :D
In short, you can do whatever you want. Heck, if you find that the settlement you were going to place in during exposition doesn't have silk, but the neighboring one does, nothing stops you from moving over there and spending your EP there instead :shrug:
Whether you want to just craft, or you want to grow your own crops too, or you want to move somewhere with 0 materials but a high demand for your profession, it will be entirely up to your plans. Although going somewhere where it is a primary industry would certainly be the easiest starting curve :P