all the engines are the same in the uk is200's (1gfe 6 cylinder 155bhp) , no matter which spec, unless you go for a is300, or a jap import altezza, the altezza is just over 200bhp with a beams engine (4 cylinder) but interior is not as well finished and insurance may be slightly more.
the is200 is thirsty too and you would expect more power considering, so dont be dissapointed when you realize its not a rocket ship, you can turbo or supercharge the engine but that would be expensive, air filter and exhaust modifications will only add a bit of power and responsiveness but give a nice sound with the right set up.
i would recommend if your going for an is200 to get a sport version as the main thing being it has a limited slip differential standard which gives for better control and handling :winky: and it has nice extras, metal scuff plates, half leather, rear tints etc
hope this answers some of your questions, get her bought lol