Hi,
Some great questions there.
can i add more cymbals&pads;to this kit
Yes you can. There is one additional input trigger point on the TD3 brain unit. This is currently labelled as Crash 2, but it doesn't have to be a crash cymbal which is connected to it. You could use another pad for example. You would need to manually assign the sound you wished to use via that trigger though.
is it possible to later maybe add a double bass pedal
Yes. You can do this immediately. Remember the TD3 is not supplied with a bass drum pedal as standard, so when you order your new kit, you should specify a suitable pedal then. Both beaters of a double bass drum pedal can be used on the on KD8 trigger. Infact I have even seen a triple pedal made by Dualist used on these before.
as;i am left handed do i have to buy a special pedal or if they are standard&have;to be adjusted for me to use
I'm not sure on this one tbh. I am not a drummer as such, though I do try! I am also right handed. I would have thought though that it wouldn't matter, even if you wanted to lead with your left foot. In this instance the secondary pedal would become your lead pedal. As a point of interest to raise; when you purchase a TD3 kit, the wiring loom is already labelled up for the optimum setting, with the cables measured to reach the pads in a right hand configuration. It is possible though to to setup a lefthand configuration, as additional labels are supplied so you can relabel each cable.
I hope you enjoy your kit when you get it!