If there is one thing I suggest when you first get a tank, it is to change the gearboxes to some nice steel ones with bearings. If not the new ones with bearings then the old steel ones with bushings. The zinc gears are ok, but if you are planning on taking this guy out and getting rough and dirty with it the steel gears are the way to go.
Make sure to break in your motors and apply grease as well to the gears, that will increase your gearbox lifetime as well. Don't rush into too many metal parts until you get those gearboxes. A nice 4:1 type gearbox will slow you down and make slow speed maneuverability a lot more enjoyable. After that you can simulate metal upgrades first by adding weight on the inside of the tank to see if you like it.
Here is my personal upgrade path: gearboxes> metal drive/idler wheels> metal tracks> electronics> all others
Most everyone has their own path, but I think most of us will agree a gearbox should be in the top 3 first upgrades.