I adore this coat of Molly's! I am determined to make it. It seems to be made of grey wool, and I think there is a velvet lining on the collar.
It is darted in front and has large buttons all the way down.
I love this shot of her walking through the field to meet Mr. Preston.
I believe you may use Sense and Sensibility's Romantic era dress pattern for this; however, it seems like the back seams may be a little different. Like any project, it will take experimenting and translating it according to your own taste! :-)