I don't think it's as hard as you'd think. Cheapest route would be to build a plywood tank. Large tanks also pop up at auctions fairly often and you can get pretty good deals on them since not many people want them.
Maybe a brasiliensis? I'll try to find some pics of mine when they were little.
Edit: Apparently I didn't take any good pics of them before they colored up. They did look very similar to your mystery fish when they were the size of guppies though.
My colony took forever to start, but it's thriving now. I think the key is to provide them with constant sources of food. I throw plant trimmings in mine. I did have one suddenly go aggro and kill two others in two days. But I think it was mostly due to lack of food.