I'm sure you've already done this, but I would strongly recommend talking to one or two more electricians to get their opinion on splitting the electric. It may not be as easy as your first guy makes it out to be... Think about all the branch wiring that is likely shared from one apartment to the next. It's going to require a lot of new wire pulling and without access to common junction boxes you may be in for a headache. Just food for thought.
To the question at hand, splitting gas should be significantly easier. Depending on the number of appliances and types, you have a maximum of 4 locations per apartment to take the gas to; furnace/boiler, oven, hot water heater (if not common) and gas dryer (if provided). You mention wanting to keep radiant heat but also wanting to purchase 4 new furnaces. I assume you mean boilers? In my opinion trying to retrofit existing radiators from a common system to individual isn't worth the effort. Since you mentioned the mini-split I assume you're open to considering a traditional split system as well, and one other option is a "packaged unit". I've recommended them before on the forums because I've used them and like them so much. Take a look at the link below for just one of the manufacturers. They require ducting like a traditional system but the benefit is, as the name implies, it's one piece of equipment. And although you may not need it as much in upstate NY, it has the option to have cooling as well. Something to consider.
Edit: won't let me post a youtube link. Lookup Magic Pack, Comfort Pack, Condo Pack, etc.