The best solution is to use a helicopter to lift and set the units. The upfront cost will be more expensive than any crane you could use, but it will save many hours in labor, and will get the job done quickly, safely, and professionally.
You could temporarily demo the gas/water piping and/or build ramps over them and use a gantry with big inflatable rubber tires to move the units from the edge of the roof to their final resting place. You'll still need a crane, and everyone on the site will have to work twice as hard to get the job done in the time allowed. This will also result in you having to purchase a lot of specialized equipment to move the units around, plus a lot of extra labor hours. This is not the preferred method. It may not even be possible at your site.