Pls consider the following aspects:
A1. As per the facts, the NRI individual has purchased the land, Plant & machinery through a slum sale agreement. So was it a transfer of business undertaking of the private Ltd company?
A2. If so, now whether the said undertaking is transferred as such to your client firm?
A3. them, pls consider the eligibility for exemption for services by way of transfer of a going concern as a whole or part thereof. May be some tax planning can be done in my opinion.
B1. If the assets are sold individually, then you need to ascertain the values of the same, which is not given in query.
B2. Sale of land will not be subject to GST as its Sch III item, But it shall be exigible to stamp duty. Consider the Income Tax TDS and Capital Gains provisions for the transfer.
B3. The other assets i.e. plant and machinery will be taxable supply. Practical stand is for the NRI to take GST registration If the value of same exceeds Rs.40 lakhs (assuming intra state supply) and pay GST on the same.
B4. In any case, I do not see any RCM liability in hands of firm as there is no import of services here.