Hello and welcome to APC
One reason a under gravel filter is not recommended for a planted tank is if you use the correct size (2-3mm) gravel it will fall through the holes of the filter restricting water flow. Another reason is the roots of the plants will get entangled around it, making it very difficult to remove the plants in the future.
Since you will be using C02 I would recommend a canister filter over a HOB. With a HOB you will get surface agitation causing C02 loss.
In the beginning it is always best to start out with a heavily planted tank that contains a lot of fast growers. This will help to consume nutrients that the algae also feed on, making it harder for algae to get a foot hold.
What type of hood are you getting?