Hi! I would love to help! I have a 75 gallon with 5 fancies in it, but also have a 100 gallon Rubbermaid stock tank in my basement that currently houses my 18 pond comets for the winter. In warmer weather, they live outside in two smallish ponds. I also have cats...7 of them. (OK, only two are mine, the rest belong to my roommate.)
My first thought is that a whiskey barrel with a hardsided liner would be prettier than a stock tank, but also much smaller and your fish would be more able to jump out since the water level would need to be closer to the top than in a stock tank.
You could cover it with something to keep the cats out, but unless your cats are serious hunters you probably do not have to. I keep the stock tank half covered with a board, and our cats like to sit on the board and drink out of the tank. My youngest cat used to like to stroll around the rim, but she fell in once and that was the end of that.
Re: lighting and planting...can't help you much there! I have not tried either, as my set up is just a hibernating/holding tank for the winter. It is in my basement, and it gets cold, and dark, down there. It's very functional, but not pretty. But before you feel bad for my fish, here is a shot of the upper half of their summer home. There is a second pond out of frame at the bottom of the waterfall...
Re: stuff you MUST have...a filter is at the top of the list. I made my own with some craft mesh, a smaller Rubbermaid storage tub, and a small pond pump. It was a bit of work, but I enjoyed the design process and it does a good job. Next winter, I will be upgrading to a DIY "out of the tub" type that is sometimes called a "skippy filter".
OK, enough for now. Hopefully someone else will chime in with plant advice.