For low memory usage you should use low resolution HDRI map(just change size in photoshop), you do not need it in high resolution as you override background and not see it. It also reduce render time.
STP and STL file open fine in 3ds max directly, you can choose quality of mesh at import or even keep them as nurbs model.
16gb is should enough for product renders. But if you render very big resolution for example 10k this may be problem, as whole pixels stores directly in VFB during rendering and not compressed it can take several GB, so if you have warnings about low resolution and rendering is very slow, you can turn off VFB, also use option low memory in system settings and dev settings, this save a bit RAM