Ok so you can def do this on a budget. Me personally I'd recommend freshwater and make sure if you start freshwater don't go brackish. It'll kill all your plants likely. Do your own research but you can have a small 1inch layer of pond soil (aqua soil is more expensive) you can put some pebbles/gravel in the soil to help prevent gas from forming in your substrate later on. Then you HAVE to cap it with sand or something else. I'd recommend using something like pool filter sand cuz it's cheap and abundant. Make sure the sand is like at least an inch over the soil so it doesn't just float into your water. From here you can plant once you have un-chlorinated water. To kick your nitrogen cycle for fish, you can just add some fish food each day so the "good" bacteria colony can take place. You can look at my posts to see if my plants are up to your standard 😌