1. You need to seal it in with 100% silicone. Now, this is important. you just need to run one bead on each side of the divider. Allow it to cure fully (24 hours minimum, could take longer) then fill up your water section and see if you have leaks. If you have a leak: DO NOT just add more silicone, it will NOT seal. If you have leaks, tear it apart, clean everything up. All silicone must be removed and do the procedure again. Silicone will not stick to silicone, this is why it has to be done in one step.
2. You can probably add a pair more into the enclosure. Make sure you quarantine them for 30 days before adding them.
3. For more exotic plants, look for plants for dart frog enclosures. I know there are a few dart frog suppliers in the uk, just not sure who they are. There was a thread started around here somewhere listing suppliers in the uk. Keep in mind that the more "exotic" you go, the more the plants will cost.
4. A simple piece of aquarium tubing placed in the tank before substrate is added will allow you to drain the excess water. Again, there are a few threads about that subject in the Viv section, you just need to find them.
5. Lighting can be any type fixture you can afford, but the bulbs need to be in the 6500k color temp range for optimum plant growth.
Education is the most powerful weapon we can use to change the world ~ Nelson Mandela