A. Depends how powerful a light you get. Six 40W tubes would be about 240W, so a 250W metal halide would be comparable and a 400W would be much brighter. The cost depends on the power. 240W of fluorescent and 240W of metal halide cost the same to run, 400W costs more.

B. Metal halides must be mounted further away than fluorescents. The light and heat is more concentrated and must be allowed to spread out before it reaches the seedlings. Metal halides do also radiate more infra-red and you may have to use a fan and water more often.

C. Not sure what your question is. Any artificial light bright enough to raise seedlings produces way more heat than you need to keep them warm, the only question is whether your setup traps too little, too much, or just the right amount of heat to maintain the temperatures you need. Once germination is over, lower temperatures at night are a good thing.

For the last question I posed, yes lowering the temp after germination is a good thing, but when the heat mat would be off after germination and the lights on 16 hours, the other 8 hours I worry about that sun room in January/February is way below freezing.
I don't have that issue in the basement, as the lowest recorded temp when light/mat are off was 57 degrees with a plastic cover over the seed box. Ambient temp in the room about the same.

Even with a heat mat, you'd have a tough time raising seedlings in a room that gets below 45° or so. After germination, the mat might keep the roots too warm and the tops of the foliage too cold. And there's always the possibility that a power failure for an hour or two overnight could kill your plants (though you could buy a sensor to warn you of a severe drop). If you can't insulate the sunroom, I'd either get a couple of space heaters or avoid using it during the winter.