As a rough guide, if you are doing one-off items then probabally a manual mill would be OK if you plan to do production of any considerable quantity then the CNC will shine.
If it is something that you intend making a buisiness out of, if only part time at first, and you have the room, go for the largest mill you can afford, especially if you want to machine stainless e.g. minimum 36"x12" table Bridgeport knee mill etc.
There is a lot of work converting a manual mill to CNC, machined pre-loaded ball screws alone cost a bundle, I would not reccomend going with rolled BS with no preload. There a lot of small CNC mills that come up on ebay etc and usually it is alot cheaper to bring those up to snuff than starting from scratch.