i operate from several different geographic locations ,regional and international and do not want to carry around a macbook everywhere, regionally i use a push bike so it gets too bulky with a macbook in my pack and i like the faster machines for graphics, i was thinking of using one external portable ssd hard drive to keep all files in, especially the emails using entourage or mail , and all the attached files with it. a small light loaded enternal ssd in my pocket or backpack would be great. 

Even better would be a set up where i could use anyones imac, macbook , power mac and boot it up using my ssd with my mac home file located on it , and automatically used and saved to. 

if this all can be done, then backup would be copying the ssd. as it will probably use firewire 800 can it copy by daisy chain two ssd externals and manually copying it all , or would there be some program that could back up only what in new or has been added too. i would need to occasionally transfer to archives old rarely used files to keep enough space free.

this is the dream, to always have all my data available .. in that one flexible ssd drive. 
can it be done ? is it too hard ?