Radiant heating does not need to be limited to the floor. You can add radiant to walls and ceilings. There are also pregrooved panels available that have channels cut for the tubing and reflective barrier built in to reflect the heat. Hella expensive though!. That being said (and having done radiant heating and loved it) this is what I would do in your situation.
- Get an energy audit done. A lot of utility companies will do it for free.
- Good old fashioned air sealing. Drafts destroy comfort no matter how good the heating system
- Look at areas that can be insulated easily and do it
All of that can be done easily and cheaply. It also can make a huge difference on indoor comfort.
To retrofit with radiant you'll need a heat source. A standard furnace will not work, which means a boiler or a hybrid hot water heater that has space heating capability. Those are pricey. You'll nee to run tubing through joists bays or into the panels I described above. Again, this doesn't have to be into the floor. You need pumps, a manifold and a controller.The system also has to be sized correctly based off heat loss calculations.
All of this is doable. But it will be a big project even for just a couple of rooms.
You'll also end up with kind of a Frankenstein heating system. It may work fine, but if you ever sell...explaining all the different ways you heat the house to a buyer could be challenging.
As a fan of radiant heating I'd encourage you to research it carefully, but in the end, in my limited opinion, retrofitting the whole house with a split is going to be the way to go.
But if it turns out radiant makes sense...it's doable and there are companies like Radiantec that will help with all phases.
Good luck!