i was going to keep a payara; but it's impossible to feed them.
they don't like goldifhs or pellets. they need piranhas, seriously.
anyways i heard they die at a small size to but in the wild they get well over 3' so it'll outgrow the 180gallon.
and where are you going to buy them, online or local fish store?
they should be around the $50 mark if they are doing well.(which is pretty much hard)
i know a guy in japan that has one around 1' but i don't know if you speak japanese so...
plus they should be kept in schools of 6or more so you need at least 1000gallons of water, maybe more.
get alternates, payaras are too hard to keep.
i liked payaras of their size, body shape, and teeth so i guess you like them the same reason that i do.
great alternates for a payara could be arowanas, they have the same body shape.
for their fangs; you could get gars, toadfish, piranhas.
and the size, you can get something big like a irridescent catfish, pirarucu and alligator gar(if you have space), red bellied pacu, clown knifefish.