GREAT price, but the from a tailor's points of view, could be fiddly, depending on what you need re-shaped.
I only buy pre-loved clothing that I alter and re-shape, and in some instances, if I love the fabric sufficiently, completely take apart and re-make (I am about two to three sizes smaller than what is usually on the pre-loved racks).
Although the jacket you love is absolutely DIVINE, a 42 going down to 38 with all those panels, would, I imagine, be fairly fiddly if your tailor has to alter the following: shoulder width, waist width, sleeve length, back darts, and finally the lining so it does not bulge... I hesitate to think what the tailor would need in terms of time.
42-40 is not so time consuming for the tailor - I think that might be a simple case of the single side seam being sewn in for you.
If you are the right height for the jacket and the shoulders are fine on you and sleeve-length perfect, then you may just need the tailor to run a seam in at both sides (or in the back: I can't see what the back looks like, so your tailor would best be able to tell you). This won't be anywhere near as time-cosuming or expensive.
Anything is "alterable" in my view: the questions are
(1) how long it will take (time=$$)
(2) will the tailor be able to maintain the jacket's original shape (it looks almost bell-like at the bottom - so pretty! Plus there are pockets that seem to almost go all the way around to the side seam)
(3) will you like how it looks after it has been altered, if there are any unavoidable changes to the shape and structure?
(4) is the tailor good with leather alterations (some tailors are fine with leather, others are not so great)
(5) also, from what I can see on the picture attached (is that you in it? If it is, there's no need to alter it, as it fits perfectly) the front breast pocket zips are again quite close to the sleeves: so if the tailor has to pull in the shoulders to fit you, the zips may appear out of proportion in that they move even further off to the sides than they already are
There are some very good tailor: maybe you could get a recommendation for a good, reasonably priced tailor and go from there?
Or, you could purchase the jacket (it is very well-priced), take it along to get some quotes from various reputable tailors and go from there. If you can't find a good tailor, you may be able to re-sell the jacket again, since you have not touched it, and it really is gorgeous!
Finally, what if you take this picture to a tailor, tell them where it's big on you, and see if they are happy to tell you whether they can do it for a price to suit your budget?
Hope this helps!
And, if you do purchase it and find a great tailor to alter it to your satisfaction, please post your modelling shots: I'd LOVE to see you in this gorgeous jacket!