I understand a project of this size would require a full time team of Architects, Urban Planners, Commercial construction team etc in house or hired out with prior experiences.  

Are you an architect/engineer?...

I'd start with a local RE attorney, and try to get a good local architect or land use consultant that can help with possible after the fact permits. 

It just needs some work or maybe an extra bathroom would be the key.”As an architect, I’m happy to put ~$50K of work into this if I can finance it. 

I've spoke with a few architects so far and the going rate for an architect is around $10k.

That technically triggered the requirement to get a COO before they moved back in.... but no one paid much attention to this, not myself, not the architect, not the customer, and not even the officials at the little high-end SoCal beach city.

The Act requires all newly constructed multi-family dwellings of four or more units intended for first occupancy after March 13, 1991, to have certain features: an accessible entrance on an accessible route, accessible common and public use areas, doors sufficiently wide to accommodate wheelchairs, accessible routes into and through each dwelling, light switches, electrical outlets, and thermostats in accessible location, reinforcements in bathroom walls to accommodate grab bar installations, and usable kitchens and bathrooms configured so that a wheelchair can maneuver about the space.Developers, builders, owners, and architects responsible for the design or construction of new multi-family housing may be held liable under the Fair Housing Act if their buildings fail to meet these design requirements.

I recently closed a building with 24 residential units and it is more than 100 years old.I have talked with a designer/architect about interior design.His quotation is around 21k for all the units.Is the quotation reasonable?

We are architects and have inspected the property, deed restrictions etc. 

@Josh Spears - you will need to talk to an architect first.