Any substrate can work well for a planted tank. If you want you can still use the plain gravel you have if you like the look of it. If not, you can swap it or you can mix substrates without any issues.
For cheap substrate, pool filter sand, black diamond blasting sand, normal aquarium gravel or normal aquarium sand can all work.
You would have to add a little fertilizer for the plants if using an inert substrate. Thrive or Easy Green are cheap and quality all-in-one fertilizers.
If it were easy we wouldn't be here