Hi Roland and welcome to the best Lexus forum there is. Excellent choice wanting an is250 as your first Lexus. Browse the 2nd gen IS part of the forum as your question has been asked a good few times and you'll find a breakdown of spec differences. You are typing SE and SE I. SE L is the one fully laden like wood, rear blind, ML system, rear camera, satnav etc. There are a number is is250 fir sale on auto trader which I'm sure you've seen. Yes some specs of cars are confusing. se-I se se-i seems odd but there you go.
Look for the threads what to look out for buying an is250, there's tons of info. Happy reading.