We have a loan for the purpose of purchasing land only (no structure, no intent to build, no repayment using refinance for construction) secured by the land as well as an abundance of caution third lien on the primary residence. For purposes of Reg B collection, would GMI be collectible? If so, would we report on the LAR per Reg C or mark all GMI fields as “NA”?

There is no such thing as abundance of caution when it comes to Regulations B, C, or Z. Including ATR/QM, HPML, HOEPA, etc.

Since the primary purpose of the loan was to purchase land would would we report GMI? We know it is Reg C applicable because of the third lien but are unsure whether we would collect and report demographics.

You would need the GMI/DI assuming they're natural persons. As Randy indicated, there is no exemption from HMDA for abundance of caution, so all aspects of HMDA apply, including gathering and reporting DI for natural persons.