Now that you think you have enough yarn and know your gauge, and I hope you did a good size swatch to judge texture and colour as well as gauge. We need to decide how you want to construct the coat. I am sure the original was constructed quite simply: worked in garter stitch in one piece to the arm holes then the sleeves worked separately and sewn on at the end. This is easy enough to do, and if you decide to do it this way, I would suggest a good technical design book, such as the Vogue knitting: the ultimate knitting book, I love this book, and I really wish I had my own, but instead I monopolize the library’s copy.
If you go this way, then my advice is likely of little use to you. Other writers have made it all quite clear in other places. I however would like to suggest a slightly different way . . .
Please bear with me here, what I would suggest is constructing the coat in one piece from the top. This has several things to recommend it: most importantly you won’t run out of yarn before you finish the second half of the last sleeve. Perhaps it is just my bias, and there are ways to measure how much yarn you need: e.g. work a square of a set size, such as one foot square, weigh it on a good scale and divide it to get the weight per square inch and multiply it by the number of square inches you have worked out your sweater will be, but I never do this, and working from the top means you can just continue until you are happy with it or you run out of yarn.
I would cast on for the little mandarin collar, then cast off and pick up the number of stitches to start the yoke, you could have a random increase of stitches and have a round yoke, make raglan shaping or work out a different shape and increase for the sleeve it in a line where the sleeve seam would be. I haven’t worked out exactly how this last option would work, but I think it could and it would minimize the difference between the shape of your sweater and the original.