You have a lot of good ideas here and it sounds like you are preparing thoroughly.
When I buy online I use azponds.com. Huge selection and good service.
The space you are working with is fairly small- don't try to do too much. Don't push the height. Be conservative and then do it really well. If you try to push the height too much in that area you will end up making mistakes that lead to leaks. Create spaces in and around the feature for plantings. You can hide a lot of mistakes that way.
Have fun and let us know how it turns out- if you get it.
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