Preiser has a rather large range. So which 1/35 figures and what are you trying to achieve with the conversion?
The "Academy" figures are nice in terms of proportions if you need a bare figure (and who doesn't need a bare lady? :stinker ) to start your sculpting adventures. That said, you'd do just as well using the academy figures from Helo, Michael Roberts, Colorado, Andrea etc. The key difference being Preiser presents the completed human form whereas the others offer one a head, hands, feet and the wire/etch body frame.
Again, it all depends what you're wanting to do. If all you need is a human form you'd probably do just as well scraping down an old Tamiya figure and adding the detail.