If you are new to the hobby, especially the nitro side of it, I would definitely stick with the stocker engine right now. The Associated .15 is a proven workhorse, and is all the power you need for now. They also hold a tune much better than a racebred engine. If they are kept tuned well (not to hot), they can last 4-5 gallons of fuel.
I wouldn't invest any money in having any porting or anything done on it. When you are truly ready for more power, your money would be better spent on an engine upgrade.
The engines you mentioned are all O.S. brand engines, just different models. Some are still available new, and some are discontinued but still available off of ebay from time to time.
When you do upgrade, I would recommend the OS TG, as it is a great intermediate engine, holds a tune well (as do all OS engines!), and will last for 5+ gallons if kept tuned well. It's hard to beat an OS engine, especially for the price.
As far as carbs go....it's a matter of preference for most. Typically, slide carbs are better suited to on-road applications, as they are more low-end power oriented, where rotary are mid-upper powerband designed. I have however ran slide carbs on off-road stuff in the past with no problems. Many people still prefer slide carbs for off-road. When you replace the engine in your GT2, you will need to have a rotary carb, as that's the way the truck is designed.
Hope this helps. Welcome to the hobby, and remember it's supposed to be for fun, so don't take it to seriously...lol.
Let us know if you have anymore questions......
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