property investment ,what taxes are due?

I have been approched by a builder who wants to offload a property but he wants to keep an equity stake in it of one third ,the property is in a good letting location.the deal would be as follows,

value the property
new mortgage with both names on
legal agreement on my two thirds and his one third share
timescale when to sell etc

can anyone tell me what stamp duty I would have to pay,and does acquisition tax apply in this case,has anyone completed on a deal like this
thanks for any responce

thanks for any response spellings

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