Having some car problems can someone help?

i just bought a 90 isuzu trooper for 500 off this guy knowing he didnt get it smog when i took it to the mechanic he said one of the cyclinders isnt fireing its just dead but i was able to drive it 86 miles from the guys house to mine and 8 miles to the mechanic no problem what could be wrong. An also when idle its very rough for some reason could i get some advice on that