Completely new heating system ideas?

[Jamie Brayton]

I'd like to make an offer on a solid 3 family that is a major rehab. I'd like to avoid tearing it apart more than I need to, as it does have cool, ornate ceilings in most units. 

 It had a mix of individual gas, wood burning and electric units scattered throughout and these have been removed. There is no duct work and no lines for boilers. This is a BIG 3 family and appears to once have been two properties merged and put on one deed-- that feels important because it presents new challenges in running lines (a sturdy brick wall divided two of the units, no shared basement). 

What are my options here? I'm willing to get creative, but I'd like to install an efficient system that won't result in high energy costs for tenants. 

ETA: The less invasive the better. While this is a big project, there are some really interesting features I'd love to salvage. At this point, I think I'd choose higher costs to install over a cheaper, more invasive option.