leaking shut off valve (steam)

shawn indrerd

i recently bough an old house with good old steam heat. as the winter is progressing more and more of my shut off valves are starting to leak.
i think they are 1&1/4" valves. however, some are on the side of the big old cast iron radiators with the knob on top, and some are underneath behind a panel in the fairly newer recessed radiators with the knob on the side.
so here are my questions:
1- should i first attempt to repack the nuts before i change the valves completely?
2- when i repack them, what type of compound should i use?
3- how do i make sure i purchase valves that fit? will i need any spuds, bushings, nuts etc?(i can buy extra pieces if necessary and return them if they are unused)
4- is it possible for my valves to need different sizes on both ends?
5- do i need different valves for the different radiators described above, or is it the same valve turned on its side?

for all this, bear i mind i have no previous experience with this specifically, but i am very handy and have done various plumbing and pipe repairs in the past.