I personally think you would be much better off spending your money on a heater and maybe just a few rocks and a plant.
The heater doesn`t take up a great deal of room, it`ll be stuck to the backside of the tank and you don`t have to buy a huge one, it`s much better for your fish to have a heater than to put lots of rocks in there.
A nice plant and something small for your fish to hide in if he wants to AND a heater are the basics. If you feel you have room for a few more rocks too then put some in. 8)