The correct answer is 'any'. In negative contexts (sentence with 'before' implying he hasn't purchased yet), 'any' is used as a pronoun meaning 'any one' or 'whichever'. 'He wanted to talk to someone knowledgeable before purchasing any' means before buying any computer. 'Ones' would need a determiner, 'many' doesn't fit the singular context, and 'other' requires reference to specific previously mentioned items.