welcome to TPT forum
What to use is all up to personal preference, and your wallet....
If money is not an issue, then seek out ADA substrates. I believe there is a good store in the UK called the green machine, ADA store. yes, here it is http://www.thegreenmachineonline.com/
As for plants, almost entirely up to preference again, but with a few plants being super expensive because of their rarity. That website, green machine, has a good list of articles and even a plant directory to help you figure out what you want.
Now the expensive part is whether you have the money to buy a full out CO2 setup that would work for your huge tank, which would probably start out at $500, US dollars that is.
Good regulators can run a high price, and again green machine will help you out there.
figuring out plants, layout, and the physical needs of the tank is the main bulk of the task. after that, once the plants are established and the tank cycles you can start figuring out what fish to add. but with plants its usually limited to just south american fish, or shrimp (asia/south pacific). once again, green machine will help you out.
if this is your first planted tank, and you got the time and money for it, then go all out. buy a good light, buy a filter system (for the sump), buy good heaters, buy good substrate, buy good plants, and certainly do a full CO2 setup. Before deciding on these things, take a visit to the green machine if you can, or maybe just call them. Don't take this as me saying we won't help you out here, but you have a fantastic shop there in the UK to go to.