I have a 1 garage attached garage which was built afterhouse. First part of garage has plaster walls that is crumbling off with the ath showings. There is a wall n mddle going into another garage mainly built for storage someoe toldme they can take paster off rebuild walls, take out windows, pour cement for foors take out middle wall andmake it a true 2 carage. Quiote aaround 16000 is this feasabke or should I do a tear down, My entrance to basement is in garage. So I need it attached. Garage is probably around 100 yrs ld

That is an onsite choice, Often buildings that old are made very solid.

But you should be getting at least 3 estimates and opinions on this from local contractors.

For $16,000 I would expect a nice new 2 car garage with lights, outlets, and a garage door opener. Since you appear to be in a northern climate, I would suggest splurging a bit and get the new garage insulated with insulated doors.