Get the heater and the tank first.
You can buy the gravel and decorations later on.
Also, if you get a little creative, I'm sure you can find a few things you can use in your tank that you already own.
Also, check out craft stores for substrate and decorations. They are a fraction of the price than petstore stuff. There are river rocks, plants,crystal and plastic beads in my local pet store that can be used for tanks. You just have to judge what CAN be used. Stay way of painted stuff. If you're looking for plastic items, for example, look for the item that had the plastic dyed before being produce over the one that was produced and painted on.
Hope I make sense