Some Beginner Questions Options

[MyFirstCar]

Hi, this is my first post on this forum, as well as my first car so bare with me. My first questions is about the engine. From reading this link http://www.6gc.net/information/7 I understand that I have an 5S-FE engine. But somewhere else on the forum I saw terms being used such as these (AT200, ST200, ST201...) What are these numbers and how do I find out (by looking under the hood if possible) which one mine is? I searched and found that the AT is related with the ST trim, and the ST is related to the GT trim but i couldn't find out what the numbers meant.

Also, I bought this 94 GT with 117000 miles on it for 3000. The car is in perfect shape. Will this car make is past 200,000 miles most likely?

And my last question is, is it worth it to put out money for upgrades on a car with 117000 miles (I plan on keeping it until it something really bad goes like trans). I'm talking about stuff like a full exhaust system. I not an idiot and I am not going to race it. I just want the experience of working on the engine as I am a car noob.

I appreciate answers to any one of the questions.