You are correct.

And the ej20 takes 91 octane or higher.

 

Your assumptions are correct. The Impreza from 2002+ has only had a 2.5L N/A, a 2.0L Turbo and a 2.5L Turbo. The only models that have a hoodscoop on them are turbos in those years. In previous years, Outback Sports and 2.5RS (based on the Impreza) had hoodscoops but were naturally aspirated.

What do the headlights for the Impreza wagon w/ hoodscoop look like? If they are rectangular, then it is a 2.2L N/A and they got the year wrong (its a 2001, not 2002) and it is an Outback Sport. If its a bugeye, then its either a 2.5L N/A (stock w/o a hoodscoop) or a 2.0 Turbo WRX wagon. A hood scoop, of course, can always be added later. No way to tell unless you look under the hood.

And yes, the turbo motors need a Premium fuel. The 2.5L N/A model needs only regular fuel.

 

I wouldn't use the hood scoop as a basis for designating one model from another. While the new 2.5RS Imprezas don't have a scoop from the factory, it's more than easy enough to put one on.

 

Its doable, but the 2.5L block isn't the same as the STi block, so you won't be able to get lots of power out of it safely. I've seen plenty of people that have successfully turbo'd the 2.5 N/A block, but its not that common.

 

turbocharging a stock engine, for 99% of people is not a wise idea. Unless you can get a really good deal on the parts, and plan on doing all the labor yourself, it's generally a pretty expensive and time consuming project to do right (several thousand dollars at least). It's also difficult to cost justify, since, you rarely gain any resale value by modding your car. As much as you see people asking high prices for heavily modded cars (e.g. aftermarket turbo, S/C, nitrous, etc), actual selling prices on cars like that are rarely more than slightly above book for an un-modded car, even if it has 5-10K of mods on it.