Buying my first Industrial cnc machine Atrump BF6C

I came across an opportunity where a college has a nearly unused Atrump bed mill with 5hp bt40 spindle and 50” table and Centroid T-400 control. I’m told it is 4th axis ready and wanting to know if there is anything I need to consider when adding a 4th axis? The plan is to build one using a harmonic bearing drive and I’m a little unsure what would be the ideal gear reduction? I’d like to be able to have it operate within the scope of the machine (up to 120ipm speeds). Size of bearing 32? 40?. This is new to me from hobby machinery so rather than buy several components to turn around and buy something else is something I’d prefer to avoid. If everything goes as planned it should be delivered here end of next week. The sooner I can get the ball rolling the quicker I can have it up and running the way I need. Also any other things I should consider about this machine or need to know that may be a historical issue, value, downfall, benefit? Thanks for any insight you share.