First generation X6 upgrades ?

[andys]

I have the very first generation X6 and am thinking of upgrading it.
Which upgrade pack is the best value ? I saw there is one with and one without the chassis. Do I need the chassis ?
Also what is the benefit of the 4 gear transmission. Does the 4 gear mean you need to run the motor/speedo in reverse or anything like that ?
Lastly which rear axles are best and where to get them and is there anything else that's a must have.
In fact would i be as well to just by the squared kit ?
Cheers.
Andy