Hi mimi,
The pond looks really nice and extremely well thought out. There are a number of schools of thought about the best way to do things but ultimately it will probably boil down to the amount of time and money you have to spend keeping it up and running. And I guess in that only you will be best placed to answer this question.
Here are a couple of suggestions at each end of the time/budget spectrum ( I assume you'll probably fit somewhere in the middle)
Cheap non-time consuming way to do things
With the depth that you have in the pond and the number of plants already present, it will be quite able to sustain a number of smaller goldfish without having to do much else other than feed them every couple of days. The introduction of a fountain/waterfall will undoubtedly help on hotter days by keeping the temp of the pond cooler for longer and by adding additional oxygen to the water.
More expensive/time consuming way to do things.
For the cost of not much more than £100 you could install a filter with UV and pump (you'll need some sort of pump for the fountain/waterfall anyway) - The pond is deep enough to be able to accomodate more expensive fish e.g. koi etc and with a filter you'll be able to stock the pond with a lot more fish than otherwise. You'll probably need to clean the filter out every couple of weeks and if you're going to buy fish such as koi and have more fish in there, then they'll probably need feeding more often, especially on hot days. (probably twice a day) ***Remember though that frogs/toads etc are generally not suited to ponds with filters/pumps in them and although they can/ will live in there, it is not their ideal living conditions)
Whichever path you chose I'd definately be inclined to do the two following things:
1) Only ever introduce a couple of fish at a time, even with a filter the "good" bacteria present will need time to catch up with the amount of waste it is having to cope with. Overstocking will cause increased Nitrate/Nitrite levels and the fish will die.
2) Cover the pond with a temporary net that can be removed as and when required. Predators such as herons/cats will be quickly on the scene if they know there is a meal going cheap.
Hope this helps
Tony