How to estimate costs to build apartments into gutted warehouse?

[Jason Turgeon]

I'm looking at a property in Berlin, NH. It's mixed-use, 36k sf total. There are several rentable garage bays (about 50% rented), a storefront (vacant), and one finished 2-bedroom apartment (currently rented). Upstairs are two floors ready to convert to apartments.

The current owner claims to have $600k in work into it in the last 10 years. It has a new roof, new windows, new sprinkler system, newer garage doors on the bays, and newer electrical service and panels. He's already gutted out the top 2 floors, which look like this:

Current owner had planned to put 7 units in the top 2 floors.  Assuming he was on the right track (questionable), I need to figure out what it would cost to do that.

With a shell like this, what are some rough ballpark numbers to build it out to a apartments? I have seen estimates for new construction of $125-$200/sf, but that includes the actual structure, which I already have. Could I do a build-out of the gutted space for $60/sf? $100? Thinking of keeping it loft-style, big open spaces except for bedrooms and baths, trendy but low-cost finishes, hopefully 2 beds/1 bath per unit. Minimalist kitchens along one wall with an island to separate the kitchen from the living area. 

I believe there is probably something like 7000 sf to build out. I just need to have some rough idea of how much it will cost to convert that to rentable space so that when I go to interview architects and GCs I am not caught completely off guard.