The first thing you should do is upgrade the gears to something else. Mato 2.0 or the 4 shaft Heng long boxes will give you way better control. Also with the better gear ratio you don't need to upgrade the RX18. it will power a tank fine if the tank has a lower then stock gear ratio.
After you do this then look at the board options. Again the BARC is lo cost and easy to install, and you have the option of adding a super sound system if you want to. It has the ability to grow with your needs. Or go with a full out IBU2 or Clark board. Any of these options though mean you have to convert to a hobby grade radio.
I may be the first one to tell you this is a must, but I won't be the only one. Inevitbly you will end up there, and if you make the decision sooner them later you will save yourself time and money. The Hobby king T6A radio is cheaper then the stock Heng long/Taigen radio, as is way more capable.