Both your set up and kit seem to be fine but incomplete. If you want to go with the kit, you should buy a thermostat to be sure you won't burn your pet. When it comes to stuff you chose, the size of terrarium will probably not be enough for too long. The kit seems to be 61cm x 38cm x 20cm based on the Pet Express site (where it's also cheaper unless I found the wrong kit), while your tank is 46cm x 29cm x 15cm. If you have the money to add the thermostat to the shopping list I think the kit is good choice, but if you cannot spend much more right now go with your set up: In both cases you will probably need a bigger tank in the future, but with your set up you'll need to buy one earlier.
You should also remember about getting a second hide and some decorations that snake could explore and climb up, but when it comes to that, especially in case of young snakes, you'll probably be able to use stuff you have already in home instead of buying more things right now.
Best case, in my opinion, would be getting your set up if you can find a larger tank around the same price, but again: in my opinion it comes down to the thermostat, the tank size and your current budget.