I did our hip barn last summer.  32' wide, 48' long.  It had a hay loft that we removed.
I love it now, but it was a task to get done.

1) It will cost as much as riping it down and putting up a seperate building of same size.
2) You will like it more if you keep that barn, but it will be much more thought/work for you.
3) You need to find someone that you trust, and is willing to work with you.
4) Don't start the project and say "This is what I need from this barn"... start the project and say "What can I get from this barn"

I wanted a 24' door by 16' tall.   Ended up with a 16'x16'.
That was what the barn could do.