if u ask me, i'd say, yes it is ok to me. but u may want to do a little google to see if this model got any problems? AFAIK, not heard anything
they are not really in the same category. yes, all are qwerty, but, C3 is basic one, with symbian S40, means, no multitask. if u are in very low budget, this is the one. no front camera, 1.3mp camera if i'm not mistaken. no 3g. the camera is but sure, it got wifi and some apps for social networking
E5 is S60, same goes with e71. but bear in mind, camera wise, E5 5mp slightly is better than E71 3.2mp. but, E71 got autofocus, while E5 not. E71 got front camera while E5 doesn't. both have 3g. if i'm not mistaken, screen colour of E71 is 16m while E5 is less. can't remember whether it is 65k or 256k. E5 got better processor than E71. but tried my father's E71, it is not so slow IMO. overall, E71 is slightly better than E5 IMO.
if u listen to music a lot, E71 is not really a good choice since it doesn't come with 3.5mm headjack, but instead 2.5mm. E71 is older model.
if u have the budget, and want a front camera, E71. if not, E5. if really low budget, C3.