I really need to know the differences between the above handhelds. And also if someone knows about their official websites then please do inform me with that.

There is very little difference between all of the above devices, they are all branded versions of a series of mobile phone made by a japanese company called HTC.

I have a UK o2 XDA IIi which is also known as a Qtek 2020i, iMate PDA2k, MDA 3 (i think).

checkout a site called www.xda-developers.com for loads of info on hacks etc for the HTC mobile PDA phones.

The XDA IIi is really only just known as that and its HTC name (creator) Alpine.
I have the XDA IIs which is known as the MDA III, PDA2K, QTek 9090, XDA III in germany.

Generally, they are all identicle as long as you are looking at the equivelent model.
There is the Wallaby, Himalya, Alpine (or Andes), Blue Angel, Magician, Wizard, Typoon, Hurrican and the new Universal.
Quite a big range really.
Check out www.xda-developers.com