Your very first ride down the steep slopes of a bona fide mountain on a rock strewn trail on an old bike will undoubtedly leave you convinced to the necessity of purchasing a strong and resilient mountain bike frame! It, as your body will attest, is going to absorb a lot of shock. If you invest in a good mountain bike frame, it will do even more: it will actually absorb, into its metal and components, much of the shock that would otherwise move readily into your own components!
Design and construction of mountain bike frames has advanced centuries in the last couple of decades, really. What is available today only remotely resembles the first so-called mountain bikes some of us rode in the early nineteen-eighties. Today, mountain bike frames are available in geometries and materials unimagined two decades ago. What you need is going to be a combination of your own body and your individual intentions. If you like the thrill of high speed downhill rides, you will need a vastly different bike than if you ride on level trails. If your preference is to avoid going over rocks, you will need a different bike than someone whose eyes light up at the sight of a boulder strewn...