I'm really not sure what someone would charge for the jack/excavation/foundation I would guess 6-10k but this is just a guess. I would ask friends, family or neighbors etc. for contractor leads. You don't have to limit yourself to the immediate area, my company came up from Newton to do our Gloucester jobs (but this was in more desperate, less busy times). I usually use the head scratching method to weed out contractors that don't know enough about any given type of job. If, when they are shown and informed of the job their reaction is to scratch their head and squint, then you don't want them because they don't know how to handle it. You want to only follow through with bids from contractors that jump right in and tell you what should be done. They should seem to have an almost immediate knowlege and plan of attack with minimal head scratching.
I have never purchased a cupola. If you look at the second property I described in my earlier note, you will see a fine cupola that we built for this carriage house. We created all the top and bottom plates on the ground, brought one up on the roof to make the bottom part (below the windows) that is attached to the roof, we used the others to build the cupola on the ground and build the roof etc. and we rented a medium sized crane/truck with driver for 2 hours to lift the whole thing up to the roof. This was a 13' wide cupola with windows, a door and fancy curved roof.
Remember that every drop of rain that is getting in and touching wood is basicall destroying your barn. I would have many contractors give you estimates and then talk to a mortgage bank. They can give you an appraisal of what your property would be worth with the improvements. You could get a 2nd mortgage or construction loan, or combo for 30k. It would probably cost you that much to build a new barn but you would have to keep it further from the lot line. It is probably worth saving to add utility to your property, to add value and to keep the grandfathered position. At the very least, put a whole-roof tarp on it and fix any drainage issues that may be leaving wood in contact with water or earth. This way if you put it off for 5 years there will be something left to save.