Brutuscz,
Thought i would post some build pictures of my pond via a link to another website. Pretty similar above / below ground pond you are thinking about. 3000g only really functional not done any landscaping / decking yet as spent funds adding a small collection of Koi to the pond.
http://www.koivista.com/display_seri...&page=0&id=123
Also,
a picture of my friend Pauls pond which is even higher out the ground with a 1m square viewing window. Paul decieded to use a concrete ring beam around his to add strength. You can see the shuttering around the pond top in the photo.
I will add from a persoanl point of view - why take a chance on wether it will hold? Make sure it does!!!