Lillys are good to add and offer shade the fish in the summer. My dad kept koi for a lot of years. For things like blanket weed he never used blanket weed treatment he use to add salt to the water that soon took care of it.

I would defo recomend a pond layout like this




You can make the filter bed and pond all from one liner with no joins. The water fall also helps to add air to the water. In real hot weather it is recomend to have an air pump as well to keep oxygen levels up in the water as hot weather takes the oxyegen out of the water

It is also recomended befor you put the liner in put a nice layer of sharp sand in the bottom and make sure no jaggerd stones are there and for the sides lay blankets all round simler to the ones removal people use it makes sure the liner is on a good smooth base as the presure exerted on it when fullof water is huge so better to be safe then sorry


Hope that helps