Whilst there is some good advice above in terms of the legal stuff I would beg to differ over putting up the building.
With a teleporter or similar lifting machine a shed of this size is pretty easy to put up. You do need some practical friends but with that 2 full days would see it done. The individual components are not that heavy and with most portal frames are self supporting once bolted to the foundations. The cladding is veryt easy if bought cut to size and attched with Tekscrews or similar.
The professionals would take little more than a day to do a shed this size - with a little more for the doors.
It is critcally important to get the foundations right though. So it you do them yourself be absolutely accurate - if you use a contractor make sure they understand the liability for the frame not fitting is likely to lie with them.
From the practical point of view have a concrete floor - they make life soo much easier, also have a long concrete apron outside and make sure there are no more than slight gradients - pulling aircraft around is easy until there is a slight slope....