are you looking to have it still fire airsoft? that complicates things a bit but not too much.
if you are ambitious enough, why not design something from scratch? use ideas from all your favorites and combine them in one.
as far as materials go, repurposing is the cheapest way to go by far. try to see things as shapes and you can find things laying about and in the dustbin that will yield lots of pieces to fiddle with. depending on how sturdy you want something, you can forgo heavy metal pieces and go plastic and wood with possibly sheet metal skin on certain spots. good paintwork can make things look amazingly real. adding real metal accents to spots that can use the help will seal the deal.
this is made from a 2x12 board and a piece of PVC drainpipe, with some bits of plywood and wire and baseboard trimboard. the stained wood stock and the black receiver are one continuous piece of wood. the front sight is a steel pipe hanging strap. all bits were salvaged. I spent maybe 10 dollars on paint.
I was told that in the movie league of extraordinary gentlemen, the big blocky machine guns the bad guys were using were AK's in clamshell cases. you could start with a fairly cheap base airsoft and remodel the outside completely.
you say you have a good bit of money saved up, but I think you will find the less money you manage to spend on the project, the more satisfaction you will have at the end.
