Budding Aquarium Plant grower
 

http://faq.thekrib.com/plant-list.html
is a good listing of plants by light levels required.
For a bit of light level stuff see
http://sankey.ws/plantlumen.html
My favourite plant so far for low light is
Cardamine lyrata, although it's more a fry-protection plant
than one for koi to show off against.
Crypts do not like disturbance - I suspect they won't
like your fish one bit. You might want to check on
Java moss and Java fern - my understanding is that the former
loves slightly acid water (I'm truing to find some here in
Canada) while the latter dislikes anything but alkaline.

I agree - plants can be just as much fun as fish, and just as
gorgeous when they do well.