car engine misfire or low idle?

I bought a 1991 Mini City E last week, when I bought it, it had passed its MOT just a couple of weeks beforehand, only needing tuning because of the emissions. On the way home I noticed that at idle it almost stalls due to the revs being so low, due to the revs being so low it makes the whole car shake, also sometime it is so low that the alternator light comes on!. At first I thought it just needed the idle raising, but I now think it is misfiring, due to the sound it makes. Is there a way to find out if it is just a low idle or if it is a misfire? And is it bad for the engine if it is misfiring as i cant afford to have it fixed just yet due to just buying the car. It has had new spark plugs and new spark plug wires, so if it is a misfire it surely wont be because of the spark plugs, so what could it be? Also if it is misfiring is that the cause if the low idle? The car doesnt have a rev counter, so I cant tell you what revs it idles at. The car has just 42,000 miles on it if that is any help. Any help will be much appreciated!