If you want to drive at 9/10s and make an occasional and sympathetic foray on track, not much.
Unfortunately, all the obvious choices do not make pads for these yet. With those massive discs, some CLs or Pagid RS* would be great. But as you're out of luck, just make sure you've got loads of meat on the pads and learn
how to brake properly (late and hard, not early and elongated).
Watch the oil temp closely... It gets hot under there!
A CG lock will
help you stay in your seat.
Drop your tyre pressures so that they match the recommended road pressures once hot.
After that, respond to what the car is telling you and be nice to it.
If you're looking at doing loads of track days (e.g. one a month) and / or want to drive at 10/10s all long, then take whatever money you have and build a dedicated car. The Leon will always be a big, heavy car to track and therefore expensive in consumables and modern cars are not cheap in the first place.
Oh, and insure it on track too. A little off could cost a fortune to fix, especially if all the airbags go off.
A £20 tuition session will make you seconds faster around the track. If you're looking for safe speed, it is always a sound investment.
Check out Dales postings on Bridge to Gantry for more inspiration / advice. That guy hustles a Leon around the 'Ring very quickly with next to no mods.