I want to ask directly , is the hdd of classic so sensitive , i mean is it easy to get damaged and make the ipod classic useless ?
I want to use it as media player as well as external HDD , but i am not sure that is it suitable for this use.
if i never drop it , does it be OK ? or else this HDD problem kind of a unavoidable problem for every ipod classic

Frankly speaking i don't want to waste my money just want to buy an ipod and use it for a long time
in this instance what do you suggest me ? ipod classic or ipod nano ?

I do believe it is a standard 1.8" hard drive. It'll be as apt to damage as any other hard drive- I don't think it has a motion sensor on it.
It should be alright unless you cause any major drops- Although the iPod Touch series can survive drops better due to Flash Memory.