Hey there Ste, welcome to the forum, good to see you are looking for a 'four.
There is plenty of info out on them elsewhere on the site, but if you want a brief overview, the st165 is the original classic, and becoming harder to find in good shape. Also the least powerful. The ST185 is probably the most common, and was toyotas most successful model. Then replaced by the st205 which was the most powerful, but also the heaviest. There are many similarities between the models, yet at the same time vast differences. If you have any questions at all, even if you think they sound stupid, then go ahead and post them up, or have a browse through the forum and you might well find an answer.
Enjoy your stay here, and make sure to keep us posted with your progress in finding a 'Four
