Algorithms to prioritize equipment renewals
Given the clarification to the question, it is a 0-1 knapsack problem (your knapsack is the money available, the value is the objective function), and look for ways to solve that one, for example following the Wikipedia lead. 

Edit: A simple approximate algorithm would be just sort equipment on increasing cost per unit (age * importance), and in that order include each if it fits (doesn't run over maximal cost). In the case of continuous knapsack (i.e., can include a fraction of a item) that strategy is optimal. Should work fine if there are many items, and the data isn't too spread out.