Adding A/C to Historic(brick) Apartment complex

[Beau M.]

Recently aquired an old vacant apartment complex. Looking for most economical way to add a/c, also updating heat/water heating. We will have to replace/change out boiler as well so looking for best options! It's a gorgeos brick building in a historical district so want to avoid window a/c units hangin out and causing an eye sore! Have considered mini-splits(continuing maintenance is a turn-off), ptac(size of exterior sleeve is a turn-off), and vtac(just started looking into this one).

Wanting to maintain the exterior appearance, use a system that will last with least amount of maintenance, and make sense for profit as we intend on holding this building long term. Any experience or advice is much appreciated!!!

Additional info: 

-building is "L" shaped and although touching-basically 2 seperate buildings-both 2.5 story garden level(3 floors); 

-1 "building" is all 1 bed/bath units, other is a mix of studio and 2 bed(18 total units); 

-property is in SD so we experience extreme hot and cold temps; 

-planning on removing lath and plaster from ceilings to update plumbing and electrical

Thanks in advance!