Arkham Horror is awesome. With just the core game you'll never play the same game twice! Victorie are hard fought (I've won about 25% of the time and every single one of the came down to our very last chance).
Yup, it really justifies the price of admission.
The expansions are all well worth the price, but the core game does not feel crippled without them (if I play with new players, we don't use the expansions).
I'd hate to not have the option to play some of the new characters but you're basically right. If you've never played the expansions you don't know what you are missing. Although it does take playing the game with some level of regularity to justify purchasing them as they aren't cheap. Or needed, like you said.
That being said, there's a lot of rules to learn, and it's better if you explain them before (rather than during) the game.
Actually, it really depends on the people at the table. I've played with plenty of folks who get overloaded quickly with too many rules thrown around at once without seeing much gameplay. Usually the basics are plenty to get this moving. Explain the goal (the closing/sealing the gate), give some options of what to do each turn and go from there is the best way to maximize one's time. Unless everyone who's new has a
really good memory there's too much to explain at the start in my experience. It's entirely possible you've had different experiences then me, tho.
I've played with about 20 new players and it's really interesting to see how other people pick up what is a fairly complex game. I have to learn to just shut up sometimes and let people make mistakes to learn on their own.