If you buy new or near new in good condition and look after it, i.e keep up the maintenance, keep it clean and keep that paint in good nick i.e keep the salt spray from being on the airframe for extended periods (you are in a maritime environment) and ideally keep it hangared or at least have a cover for it, no matter what you buy it will last you well.
I'd also consider regular (every three years or so) treatment with something like ACF 50.
By and large most horror stories come from aircraft that have been unloved and left outside to slowly rot away.
Remember also that both Piper and Cessna are unlikely to make radical airframe changes to the PA28 or C172 series. Both Piper and Cessna made major airframe changes to these series over 40 or more years ago but with the number they sell today it's very unlikely they'll make any further major changes. So long as these aircraft remain in production they will still be making parts for aircraft that go back 40 years or more.