Sure the smaller machine will do the work but the larger machine will do it easier and with more rigidity. The tooling is a cost you have to accept and account for. Why dont you just say it like this, after I get the larger mill it will take just a little longer to accumulate the mass of tooling I want. If a person has the space, electrical requirements and initial investment necessary to purchase a large machine, do it. I have never read a post yet about having too big of a machine. Did you convert the lathe to CNC? If steel is the material you will be cutting, the bigger the better.