Whatever you use, position the trim board so that there is a 1/8 or greater
gap with the vertical siding. If the trim is butted up against the siding
end-grain, water will collect and be sucked up the vertical boards, rotting
them. You might even consider a small piece of Z-flashing in copper or galv.
iron, that separates the two - then you could make the trim gapless.

Have vertical house siding that I would like to "finish off" by running
a long board horizontally along the bottom edge.

Probable something like 6" wide by 3/4 thick, in 8 foot length(s).

Can I get away with the regular Pine wood boards for this ?
I will be painting it.

Or, must / should I use cedar, or... ?