Hula hoops are a great way to have fun while exercising! I would be a little cautious about getting one that is too heavy. My experience is that heavy ones take a bit more work to keep going if you haven't hooped in years. This can be discouraging.
More work can mean more frustration = you're more likely to stop using it, but it might work your core a bit more.
Lighter (still no where as light as the cheap kids hoops that can be bought at stores. Those are too small and too light.) = you can use it longer before your waist starts getting tired/sore.
One issue with really heavy hoops is that they can bruise your waist when you begin using them. I make my own hoops. I've never weighed them, but they seem to be about average weight. The first few days I did have tiny bruises around my hoops (I have very prominent hip bones). Now even if I haven't hooped in a month or two, they don't come back.
Diameter: When holding a hoop on the ground in front of you it should reach between your belly button and chest. Many beginners start around 42"-44". Larger hoops move slower around your body and are easier to learn on.
There are lots of people that sell hoops all over the internet, but you can make your own fairly easily and cheaper with materials that can be bought at Home Depot/Lowes.
http://www.jasonunbound.com/hoops.html
There are videos all over youtube that can teach you everything from basic waist hooping to moves that let you really dance with your hoop. I love just putting on music and moving with it.