'Sale of land' is not a supply of goods / services (Reference: Serial no. 5 of Schedule - II).
So, in case of sale of wind-will along-with land, such transaction cannot fall either under composite supply or mixed supply (as there is no two supplies of goods / services involved).
Generally specking, wind-mill is a movable property as they it can be dismantled, reassembled and can be fastened to another place and thereby, falls under the term 'goods'. Consequently, sale of wind-mill should be supply of goods and GST as applicable is payable thereon.
In case of combined sale of wind-mill along-with land (i.e. against single / one consideration), there is risk of Dept. treating entire contract as 'service' levying gst @ 18% on entire consideration due to reasons listed by the Querist himself.
In this regard, definition of goods u/s 2 (52) worth noting: “goods” means every kind of movable property other than money and securities but includes actionable claim, growing crops, grass and things attached to or forming part of the landwhich are agreed to be severed before supply or under a contract of supply;
Lastly, in such type of transactions (i.e. sale of wind-mill along-with land for single consideration), it is difficult to substantiate that 'sale of land' is primary intention between parties (as these lands are generally non-agricultural and located in far-flung areas from human habitate)
Hence, it is advisable to be very careful while selling any wind-mill along-with land for a single consideration. Moreover, artificial bifurcation of consideration between sale of land and sale of wind-mill (as goods) may not necessarily solve the problem considering highlighted portion of definition of 'goods' above.
These are ex facie views of mine and the same should not be construed as professional advice / suggestion.