Adding on to pole barns is a nightmare, so pick a good contractor if you go that route. Especially if is the back end, which means that the front corner and face was built square and the fudging comes out in the back. That said, you can hide a lot with metal. Control your costs by picking demensions that meet the material length to save waste and cut time.
Get your walls up and roof on this year and worry about cement floor next year after the posts have settled. Insulation next year would be easy and that way you can run wire and hang insulation at your own pace on rainy days
Just some thoughts