Personally I'd go with 1610 it's a better all round pitch. Not sure what you mean by only buying the screws and sorting the nut later because it's normal to buy the ballscrew and ballnut together.
RE: Turning. Even thou I have a good lathe and capable of turning my own screws I dont bother and buy them pre-machined from china, It's just not worth me doing it my self for what they charge.

I'd save and buy the ballscrews before buy cheap and replace later as it's often more than just quick swap over job due to the differant way the connect or work, Plus cheap acme lead screw is rubbish and good quality acme lead is more expensive than ballscrews from china.!!