Aquaclears are the most efficient and popular form of HOB filters, and cheaper than canister. I'd recommend a Aquaclear 70, then dial down the flow so that your puffer does not get blown away. Its also big enough for when you want to consider upgrading tanks.
No favorable plants. Planted tanks are more maintenance IME, but cleaner water in between weekly water changes. You can get any plants, driftwood, fake decor, or caves as long as there lots of decor. Planted tanks tend to have substrate fertilizer, so you can use that in your own tank. Low light plants can just use any sand.
What your puffer will eat will depend on each individual taste bud, and whether your puffer can recognize food presented.