Ponies were not used until 1991, I think. I'm not at home so I'm not positive on that, but I know they weren't available in 89.
As far as the 928 goes, they are very nice cars that are in a completely different class than a Mustang. One thing to consider: Buying parts for a 928 is worlds different than buying parts for a Mustang, even if you are able to do the work yourself. A guy brought one into a shop I was working at back in the mid-90's. It needed front rotors and the cheapest I could find were almost $600 each. There are a lot of Porsche-specific tools you need to do engine work on them as well. I would highly recommend that you do some major research before buying one of those. They are really cheap right now, but you could easily sink double the purchase price into an engine or transmission rebuild on it.