To address Pole Barn guru's concern of not enough rigidness from end to end, where the railing in the loft is, you could add OSB or plywood to the the railing which would greatly brace/stiffen the upper portion of the poles at a nominal cost. Like previously mentioned, if you can afford go 32' instead of 16' you will never regret it. While removing the siding on the end & reusing it on the new end may save some money, I would be inclined to leave it in place adding doorways as needed. 1 - It will help maintain the current wind shear resistance, 2 - It will help as a fire barrier, 3 - Your current contents in the barn will stay protected while your are building, and 4 - It will save you time. Removing the old boards while trying to save them for re-use & pulling all the fasteners out can eat up a lot of time. You can still bolt new boards to the existing rafters & posts with the siding inplace. As previously mentioned:
- Your barn appears to have been very well built, you will have no problems adding on to it.
- The walls of the stall, if the posts are taken to the rafters, will add to the strength of the roof as well as the stall themselves. Restless/agitated animals can really stress walls as you know.

Using the current barn as a pattern, make yourself a list of materials, (don't forget the fasteners, they add up fast!) & have area lumberyards give you a bid. You should ask around to find out how deep you will need to have the posts in the ground in your area. To save on cost & have straight posts, many builders will laminate different lengths of 2x6s to make the posts using ground contact rated lumber for the buried portion then untreated for the above ground portion. Consider also any equipment you would have to rent, how you would transport it, fuel/delivery costs.
On a project this size labor is normally equal to the cost of materials. Example: If materials are $8,000 dollars, a bid of $16,000 would be reasonable.
If you have the time, basic carpentry skills, & family/friends to help, this can be a fun project.