I believe that the land lease will be in a superior position to the mortgages. The lease probably stipulates that when the land lease ends, it reverts to a tenancy in common.If there are no leashold comps available, then you have to use fee simple sales and determine the appropriate adjustment.
I recently did a leasehold condo and wound up comparing the unit to a cooperative, without any adjustment. Co-ops are also non-freehold estates. A co-op has a proprietary lease, whereby after a specific date, it can no longer operate as a co-op.
See this link as well: http://appraisersforum.com/general-appraisal-discussion/118259-new-construction-condo-land-lease.html
"Leasehold condominium" is an oxymoron because a condo is fee simple and leasehold is a non-freehold estate.