you cant restore the colour..... the plastc yellows because is cheap. it's not even *just* caused by sunlight - as with most plastics - apparently something abt the plastic itself makes it go yellow with age!!
yoy could get the keys from a late amiga made of better plastic.... or tippex the whole thing!
you can have up to 3 external drives - providing you can provide the juice from the power supply, which the standard ones dont.... usuallynot manaing enough for more than 1 extra drive.
i have one for my a1200 which is a PC power supply with an amiga connector... im sure you could find out on t'net how to build one of these for your a500! these provde the amiga with as much power as it asks for