They don't last forever, that's for sure. Of all the brands and models amongst my clients the P600 series seems to be the first to go to the waste bin, but to be honest, it usually takes several years. So, if you have used yours more than three years, consider the modem having earned its retirement.
I can't suggest you any brand, since the sortiment or brand names may be different in your country.
As an educated guess I suppose they would mostly be plug and play, although I've also seen some models ready configured for only one ISP. The package should tell you if a modem has preconfigured settings for multiple ISP's, including yours. Rather than hunting for the best internet offer, buy one at your local shop, they should know.