Raspbian, the standard OS for Raspberry Pi, comes with LibreOffice pre-installed. How good a job it will do translating an existing PowerPoint document I can't tell you... but on the principle it's certainly doable and should be painless, hopefully.
As to which Pi you need, it does not really matter, though VESA mounts are typically for 'full size Pis', i.e at this point a Pi3. You certainly don't need the processing power of a Pi3 though, but if you need a bunch of Pis that's still your best bet as it's not possible to get the Zero in bulk quantities (or rather any more than 1 per order) at this stage.
... not sure I have answered your question, if not post again outlining any particulars I might have missed.