One BTU = 0.2930711 watt-hour, so if you know how many BTU's you need, you can calculate how many watts that equates to, and then use your local energy costs to figure out the impact. Electric is essentially 100% efficient, a burner won't be, but could be closer to 100% than you think, depending on the one chosen.
With such a small space, wall space for the baseboards might be a big problem, and adding ductwork under the floor means it may not be very efficient if you wish to add central air later. The least expensive way to run the ducts would be to have registers in the floor.
A boiler could serve two purposes, heat and domestic hot water by use of an indirect WH. Those can be had in both vertical and horizontal formats that you'd probably be able to fit underneath. A boiler would also give you the option of staple-up radiant heat, which wouldn't restrict furniture placement or the search for a place you could put a radiator in a small room like a bathroom or the kitchen without getting in the way. A small, mod-con (modulating/condensing) boiler can often run at as little as 20% of full output under light load conditions, and this also aids comfort and economy where you can match the output to the need. The most comfortable heating system is one where it is perfectly matched to the output and is running alll the time so the heat remains constant. Forced air is probably the worst in that area, and will dry the air out in the winter. It does have the advantage that since you are running the air through the filter, you take out more dust, but hydronic doesn't stir up any that is there. 3' feet is an issue. But, you may be able to dig a well or area big enough to support that. If you already have a WH in the living space, that could be moved underneath with an indirect, and the boiler would easily fit in that space. The smaller mod-con's are wall hung, and are so quiet, you normally can't hear them run from a few feet away. I have a Buderus unit in my townhouse. The US distributor is in Londonderry, NH, so parts and expertise are close by.