Before you even go near modifying a tank, visit your local tailors and ask for a modification to your trousers, just tell them you have got into RC Tanks and they will immediately know that you require the 'deep pockets' upgrade.
You need to decide whether you are going to get into one of the following branches of the hobby:
1. Airsoft with family and friends.
2. Heng Long IR standard with family and friends.
3. International Standard IR with anyone on the planet.
Option 1 - Buy HL and run it straight out of the box.
Option 2 - Buy HL and run it straight out of the box.
Option 3 - Buy a non-running HL tank off Ebay, or from a car boot sale, pull out the original drive board, plug in a Clark or IBU2 board, and you will have realistic sounds, a lot of fancy extra functionality, and a tank that can battle against anything with a system that meets the standard.
You could easily get a tank up and running in Option 3 for around the £100 mark if you can pick up a cheap HL to start with.
Forget the metal tracks if you are running on grass, they can be a complete hindrance, depends on the tank, and they aren't cheap after you add sprockets and idlers. If you are starting out in the hobby you can always add bits like that later if you really want to.
I know plenty of people who just enjoy running tanks without any battle system at all, so it depends on your wish list really?
Rob
Here's a couple of videos. One is an HL running a standard HL drive board and sounds, the other is an HL running a Clark drive board and sounds.
http://www.youtube.com/watch?v=gwMczYLZoTU
http://www.youtube.com/watch?v=IWHsOvQ7smU