Well I have done some mods to mine, wheels, suspension, head unit, alarm, body kit to be fitted very soon and a stainless steel exhaust and rear lights.. All in all once finished it will have cost 5k, maybe a bit more, give or take a few quid.(Original estimate was 6k). I have however kept all the original parts so it can return to standard very quickly if I were to sell and it wasn't to peolpes tastes.
I think it all depends on how long you intend to keep the car for and how personal you want it to be. I think a well looked after standard Clio looks ace, but a subtle and tasteful modded one can look just as good in my opinion.
Like Dunc said, If you have the money and fancy somethng abit more personal to you, then by all means go for it. I certainly haven't regretted doing mine that's for sure.
Good luck with what you decide.