There are really 2 versions of the Extreme Kart, as far as I can tell…
There’s the World Formula – non-homologated; and the Viper – homologated. The Viper carries a higher price tag.
Homologation really doesn’t have to mean a lot, but generally non-homologated Euro karts are either older (expired homologation) designs that they continue to manufacture and often sell with basic componentry to offer an affordable kart, or they are designed as 4-cycle / World Formula karts, that don’t require homologation, and being designed for 4-cycle, may not be best suited for higher HP.
I agree with Finch that if I was looking at a budget of $2000-3000 for a chassis for TAG, you’re probably better off going with a 5-10 race old, “big-name”, well proven model.
And if you decide to quit racing after a season, your 15 race old Tony / CRG / etc. will be worth a lot more than your 10 race old Extreme… something that the less serious racer might want to consider.