Profit per hour is around 80-100k (Gold) and I personally don't have experience mining coal. I'm sure the money is MUCH more because the price of coal is higher than that of gold.
Living Rock Caverns in dwarven mining area next to varrok
when I made 99 mining on my main I made around 250kgp an hour but that was like a year ago so I dont know what prices are like now
I am going to assume your computer has a dual core 2.2Ghz processor, with 4GB ram. That would be able to run 3 or 4 decent as long as you don't do anything else on your computer.