This is one of those "whats right for you" sort of questions. It depends on what fits your lifestyle and how much care you're willing to provide.
Younger chameleons take much more attention and are more sensitive to their surroundings. And as you might expect, the opposite is true for adults, though they still do require almost daily care and attention.
I went with a 5 month old panther chameleon for a few reasons.
First off their colors when mature are absolutely stunning. Second, they're among the more hardy of the chameleons out there which is especially important to consider when picking your first chameleon. Lastly I chose that age because its young enough that I'll get to care for and watch my chameleon as it grows, but old enough that it can easily handle the stress of shipping and adapting to a new environment
On the topic of online chameleon kits; Unless you end up over paying, I'm not sure you'll find anything that's equipped with everything you really should buy for a chameleon. You'll find things like the lights or water systems coming up sub-par in most cases.
Instead I recommend doing your research and reading up on what people on these forums use.
What is your level of experience with reptiles? Kits tend to be adequate but barely so. As with most animals, the purchase of the animal will be your cheapest expense. Proper housing, lighting, food and supplements will be an ongoing expense that needs to be considered. I am a huge fan of natural sunlight and think that to get your best results you should be getting them as much as you can. Babies are cute but require lots of small food that usually grows faster than the chams can eat it all. So you end up buying quite a bit of it. If you are not squeamish about roaches, you can pick up a colony that will provide you with constant food of varying size to feed young and adult chams. It's the most cost effective way to provide a good food source.
Do you have a good reptile vet near you? it's better to find out now that if or when you have an issue. Many vets say they work with reptiles but few are any good at it and are basically learning by experimentation on your dime. So hunt around and see if there is a good one near by. There are tons of things to consider before getting one of these animals, they are amazing and should be researched well before being purchased.
As a person new to chams I would say an adult panther would be much easier than a baby or sub-adult. Over or under supplementation seems to be the most common mistake with folks that are new to chams and adults are more forgiving of that. Metabolic bone disease is an ugly thing and knowing that you were the person that caused it is soul crushing.
If you decide to take the plunge, best of luck. This forum can be a great resource for you. I will say that this is the internet so be sure to take the time to read and understand what the folks here are saying. Many times advice is given and it reads like a personal attack. The cause of most of that is experienced keepers sending out a short reply because it's a question that has been asked and answered numerous times on this forum or they wanted to reply quickly and didn't have the time to post something longer.
The search feature really is your friend but if you don't know what to ask it isn't always helpful. So ask any questions you can't locate through the search.