Hi! I definitely recommend a playpen or building your own with wire-grids (if you look up “NIC cage” you should get a lot of websites with instructions for building your own cage).
When I was preparing for my holland lop, my first instinct was to find the biggest cage I could at a petstore, but the biggest I came across was 3ft by 3ft, which is not only a little cramped for a bunny, but majorly overpriced at around $70. I bought a 4ft by 4ft playpen from petco (around $50) and later added NIC grids to make it 4ft by 5 ft.

As far as litter training, I would start by putting hay in or above the litter box (look into a hay rack or box) and pellets in a ceramic bowl in the litter box. Bunnies generally like to relieve themselves when they eat. Don’t give too much outside of cage time the first few days, otherwise she or he may forget to go back to the box. This worked great for me – my 8 week old Holland Lop has only peed outside the box once when she first arrived, and she only leaves about 5-7 poos outside the box a day. Of course, each bunny is different, so it may take a longer or shorter amount of time for yours to get it. Just remain patient and try putting the bunny in the box when you notice it peeing or pooping outside it, and you’ll probably be good to go.

On the bottom of the pen, I use poly-wall (antibacterial and waterproof) but there are many options. Coroplast (corrugated plastic), tile, linoleum, etc.
I am going to purchase natural straw mats for traction, but other options are seagrass, sisal, or timothy hay mats. Fleeces can also be used if your bunny isn’t a big chewer.

Hope this helped! Good luck with your bun

regarding a cage, unfortunately most store bough cages are not adequate for bunnies. I realize that this might be an only option for you, in which case I would go for the largest one. Or if you can attach an exercise pen to it so that the area and range can be extended. Aside from that, your other more ideal options would be a dog crate (bigger, better, but more expensive), or altogether ditching a cage and using an exercise pen. You can even attach two if needed to make a larger area. This option seems more ideal and probably cost effective compared to the cage.

as far as litter training. You have to also ask if you are going to neuter/spay your rabbit. Although I have heard of exceptions, typically it is accepted that a rabbit cannot be fully litter trained without getting fixed. In the long run, behavior wise, getting them spayed and neutered will pay off. Furthermore, if you have a female bun, getting spayed will result in health benefits that cannot be understated. As for the actual litter training, bunnies will drop their poos almost everywhere, but they tend to pee in corners. You can find out which corner they tend to pee in and place a litter box there. You might consider giving less space initially so that you can get a better idea. Once they go, you should reward them. If they have accidents, you want to soak up the mess and place it in the box to indicate this is the area that they should go in.

I built a cage with grids. I used the 800 series dog grate for the bottom and put newspaper, a shower curtain and a sheet on the bottom of the cage and I used extra coroplast to make sides and binder clipped them to the cage.