Hi Betty Jo! It's great to hear that you are progressing with this project.
I'd definitely serge the linen before washing it. I have used different types of linen for hand embroidery and they all frayed a lot.
I have no idea about the 'right' temperature, but my common sense tells me to wash it at the temperature that you want to wash it again in the future. I vaguely remember my grandmother washing linen table cloths in a very large kettle on her woodburning stove. The water was bubbling and the whole kitchen steamed-up like a sauna.
I'm sure there are folks here with much more precise advice than my rather anecdotal ramblings.