I would start with burning up all that brush since you need to do that anyway you go.
Do you plan to build it by the book or just build a dam and hope it holds water? By the book is a bigger job but is the best way to get it to seal. I know there is a lot of ponds out there that was just a dam built and they hold good but it is a gamble that way.
A 75' x 10' dam isn't a real big job. I would mostly be worried about compaction.
