Terrarium kits save you from having to visit too many shops to get all you need to set up your terrarium, but they can be difficult to find with the container design that you like.
I'd recommend searching a little on the web and browsing through the kits on Amazon but not looking much further -- your effort is better spent getting the few individual components that you'll need from a couple of different shops. This way you can get exactly what you're after!
Kits do make great gifts. I think they make great housewarming presents.
There's no standard for what's included in a kit. Some items you may find are:
Exo Terra is a company that makes kits aimed mostly at keeping reptiles. They have a large selection of terrariums of different shapes and sizes. They have a big selection of "habitat kits" which are themed terrarium kits.
This Fly trap Micro-Terrarium is a tiny terrarium that comes with venus flytrap seeds. Although the reviews I've read indicate that a lot of people don't have luck getting the seeds to germinate, it looks really great if they do.
Twig Terrariums produce some really attractive terrariums including this lovely "Grazed and Confused" Moss Terrarium.
DuneCraft produce some themed plant terrariums, like this carnivorous plant terrarium: