If you want a comparable mounting system for acme and ballscrews, then you need to machine both of them.
People have come up with a cheap DIY method of mounting acme screws to save a considerable amount of money, as end machining and bearing blocks would typically cost many times more than the screws themselves. They do this be using 1/2" ID bearings and clamp on collars that slip over the outside diameter of the screws.
You could do the same with ballscrews, but it would be much bulkier, and quite a bit more money.
Personally, I machine the ends of my acme screws, as the mounting is much more compact.
Getting the ends machined from China adds very little to the cost. The support blocks and bearings are about $50-$60. Sue, you can make your own, but how much is your time worth? Here's a little info on making then.
5 Bears - Y axis ballscrew
On a small machine like that, 1605 or 1610 would be more than adequate, be cheaper, and give better acceleration due to the lower inertia. They have a 25% larger diameter than 1/2" acme.
You seem to be spending quite a bit of extra money on a spindle, and ballscrews. So just go ahead and get a G540 kit and 381oz motors from Automationtechnologies (Keling). It might cost $100-$200 more than the cheaper alternatives, but you'll have the best package available for your needs.
No, the G250 and G251 are identical, except the G251 has screw terminals.