hi sonja. first of all, i don't think that the weight of the bike will really affect you that much being a newbie. while Klein makes some very good bikes, you might check out Trek. they make a few womens specific bikes that maybe more in your price range. there are other bike manufacturers too, like Fuji, Kona. don't go with the first thing you ride, because this is a huge investment. try lots of bikes. also, remember that the LBS may want you to buy what THEY think is right for you. only you know that, and in time, with a little patience, you will find it.
as far as frame material, i ride an aluminum, and do love it. however the ride is a little "rigid" and aluminum is known for not taking the bumps too well. that is why aluminum is good to race on. my next bike is going to be steel for sure. steel has more shock absorption, and "in general" is more comfortable. don't negate steel right away. take a closer look. i think you will find that on longer rides, the steel will be more comfortable. it won't make a difference at mile 10, but around mile 30 or 40 you will notice.
i am not saying aluminum is bad, far from it. i love my aluminum frame.
you also want to make sure your lbs has measured you properly so you know exactly what size frame to buy. and this is my biggest piece of advice that was given to me. besides the minor adjustments with stem, seat, seat post etc. THE BIKE MUST FIT YOU. not the other way around. if you are measured and are between sizes, take the smaller size of the two. you can make more adjustments to it(stem, seat post etc.) whereas the larger
of the two has a longer top tube, and that can't be altered.
also make sure you find out the return policy, so if you change your mind, you can get your money back or exchange it.
just my .02 cents.
good luck, let us know how it goes.