Hi Jack, welcome to the forum
To answer your questions (but not necessarily in the same order):
Dechlorinator
I use
API Tap Water Conditioner. This just detoxifies chlorine and binds metals. It does not detoxify ammonia so it is fine if your water company uses chlorine. If it uses chloramine, this dechlorinator splits the chloramine into chlorine and ammonia, removes the chlorine but leaves the ammonia there. In this case, you will need a dechlorinator that also detoxifies ammonia. You could try contacting your water company to find out which they use but when I emailed mine all they did was send me a water quality report which does not contain the information. I might have got the info I wanted if I had rung them but emailing is free
API TWC does not contain anything to 'stimulate the slime coat', another reason I like it.
It is also very concentrated, which means it is very easy to add the right amount to each bucket of water during a water change.
Carbon
It isn't necessary as you have discovered. It is a throwback to the days when all filters did was remove the yellow colour which builds up when you don't do any water changes. But it is useful for removing medication after treating sick fish, so keep some in the cupboard.
Lights
If the fitting is for T8s that's all you can use. However, I have LED lights that can use the fittings for supports. They have their own power supply and use plastic end caps so they can slot into the fluorescent tube holders. Google Superfish retrofit led.
But if you only want low light plants, T8 tubes are fine.
The colour is mainly down to personal taste. Some lights make everything look pinky-orange which I don't like. And with low light plants, the colour doesn't make much difference. However, if you want high-light plants you will need better lights than 2 x 18w T8 tubes, so I will let those members with this type of planting advise you for that.
Fertiliser
This depends on the plants you intend. Some grow in the substrate, some grow attached to decor.
There are special plant substrates but my understanding is that after a year or two, all the added fertiliser will have gone and you have add fertiliser for the plants when this happens. Or you could use plain gravel/sand and add fertiliser tabs from the start.
I have only plants that are attached to decor so I use a liquid fertliiser that is added to the water. Because mine are all slow growing plants I use less than the manufacturers say.
Have you read up on fishless cycling yet? There is a newer method now (see Filtration and Cycling section) which is designed to stop nitrite getting high enough to stall the cycle. The old "add ammonia every time it drops to zero" method made so much nitrite that it inhibited the nitrite eaters and stalled the cycle.
Alternatively, if you intend having a lot of plants (and I mean
a lot) you can set up the tank with plants, wait a week or so to make sure they are growing well, then add fish a few at a time. Floating plants are particularly useful for this method. Plants use ammonia as fertiliser and lots and lots of well growing plants will take up the ammonia before it has chance to harm the fish. The reason that floating plants are regarded as an ammonia sink is because being at the surface they are nearer the lights and can access carbon dioxide straight from the air.
Any other questions just ask. One or other of the members will be able to help