I never thought much about this because I collect with my 
daughter so of course they are all hers. (Some she even bought 
herself with her allowance money!) Since I have a rubberslug 
gallery, presently all the information on them is there.  I guess I 
should really keep something somewhere else.  But the price 
paid on a cel isn't often the value of the cel, so how is anyone 
not in the hobby going to know if that bargain Vash the 
Stampede isn't really $50 any more? 

As for preservation and longevity, Plastic is durable (often more 
than we want it to be!) and acrylic based paints are proving to be 
very stable in the long run.  Some pigments in them can fade, 
but most won't.  The biggest enemy to cels is light, so even with 
the best glass and materials, frame at your own risk!  The itoya 
books most people use are probably suficient for long term 
preservation . Unfortunately, the drawings are likely to be the 
first to go.  The paper they are drawn on is not acid free. (It 
would cost studios a fortune if it was!) Dampness and air are the 
biggest ememies of paper, so keep them in a cool dry place and 
keep your books tightly packed so air doesn't circulate around 
them. Even so, acids will destroy the paper on their own 
eventually. (You will probably not be around to see it, though!)  
As for the lines, I just don't know.  Perhaps, if all 
lines fade eventually, all cels will eventually be restored by 
having them hand inked.  If so, I hope I die before I have to pay 
THAT bill!

So get a short term plan going, but maybe you can find (or 
make!) someone to pass that collection on to!