Matt - I share with a passion for recreating features of historic tailoring which I feel have been lost. I have some experience in recreating a 1886 waistcoat which I had my tailor cut off a published pattern, so I can at least share with you some of my limited experience.
Firstly, do you have a tailor you have established a working relationship with? If you have that is a big step. If you can find a tailor who shares your fascination with the forgotten elegance of the past all the better.
Next, once you do find the right tailor may I suggest that you borrow your friend's coat and take it with you for your tailor to copy. It is preferable that this is not the first coat your tailor will be cutting for you as the usual problems of getting the pattern right for you the first time will always ensue.
If possible I suggest finding a period pattern for your tailor to cut off. My passion is for nineteenth century tailoring and I own several books with reproductions of period tailoring patterns. Having that makes it a lot easier for a good cutter to work off. You may have to search around - in second hand books stores, eBay, Amazon etc. I have seen you sport some amazing vintage clothes, so surely you can be equally resourceful in digging up some good tailoring patterns.
When you go to your tailor, make sure you have a little portfolio with photos and illustrations of all the details you require. If you have to verbally explain all of these things you may find only between 1/2 to 4/5ths of what you say are understood. After all if you look at the little sheet that a tailor uses to note down each customers requirements there is limited flexibility.
You will find you are going out of your tailor's usual 'comfort zone' by asking him to cut in a fashion and aesthetic which veers from the well beaten track. You have to provide him with as much helpful information as you possibly can.
As for my 1886 DB 10 to 5 waistcoat - it gets compliments all the time. It emerges fresh off the page after 120 years and trunces the lastest fads for style. But my tailor reported that his tailors found it difficult to make, even though he himself found no difficulty in cutting off a pattern.
Lastly if you are price conscious like me make sure you source your own fabric.
Good luck!