First, they are not stupid questions! I asked some of the same ones.
The I-beam will work. If it's thinner near the edges then stay near the middle. If it's over a quarter of an inch thick then that's great. One thing you might consider is getting it cut to look like an anvil. Clarification...get a horn cut out on one end. . A hardy hole would be good too. (A square hole in the anvil)
I'd stick with coal about the size of gravel. If it's too dusty then little bitty pieces of REAL hot dust tend to fly out and land on you hand. (I think that they have homing devices built in that automatically lock on target...your hands, arms, legs, face, etc.) I haven't done this yet, but I do not suggest wearing sandals when you forge!!!
I NEVER try to worry about coking. I have enough to worry about...flying bits of coal, steel catapulting out of the pot, what I'm making, what am I going to do next, where's my hammer, not that hammer-the other one, now my steel is cold 'cause my hammer was missing, so on and so forth.
Good luck! And keep the questions coming! If I can answer them ,it makes me feel smart!!! (Frosty will probably be playing with that one!!!)