by "kit" do you mean a packaged system? usually all you need to do is add a diffuser or two to increase the "gallon rating" if im not mistaken. its really the amount of co2 being injected into the tank and ensuring it is evenly and efficiently distributed that would limit any system.
Read what Rex has to say regarding planted tanks since his info is very helpful.
I'm not entirely sure but I don't think there is a "package" that you can buy from a store without getting ripped off. All you need is a Co2 tank, regulator, some sort of reactor/diffusor, and drop checker, and patience.
I bought my regulator and reactor from Rex for about 150 or so. There are other people who build them and sell them so prices vary...be sure to shop around.
I have a PVC reactor that is connected to my ehiem filter so the Co2 in injected into the reactor and "dissolves" within the column of water. It works for me but there are other ways to do it.
The drop checker is a good way to determine how much Co2 is present in the tank but its not 100% accurate. You will need to buy or make 4 Kdh water and some pH reagent. I have one but I know that in my tank, its not accurate because I have extremely high KH. So in my case, my drop checker (with 4 KdH water) is bright yellow but the plants are pearling fine and the fish act like nothing
I know this is a lot of info but I hope it helps you in some way...
CO2 equipment doesn't really change depending on your tank size.
You will need
1) CO2 tank
3) Solenoid (shuts off co2 using timer - optional)
4) Needle valve
5) Drop counter (use to measure how much CO2 you are injecting - optional)
If you buy a JBJ or Milwaukee regulator, usually 2 to 5 is included.
You can search on google "aquarium co2 regulator" to find a online store that carries the regulator.
CO2 tank can be bought online as Calros said, or buy one from local welding supply store.
There are many threads that talk about how to diffuse CO2 into water. You should search the forum for more info.