I guess I will jump in once more since NC_CAMS has not alrready
I think you will get more error from using cheap bearings for your leadscrew support, than with the use of cheapish rolled ballscrews over a few inch lengths.
If you want to do it right, and greatly reduce your accuracy issues regardless of which screw you find, use angular contact bearings on all three axes. If you want the work done for you, and an assurance it is done to the highest quality, buy duplex bearings. If you've got mechanical inclination, you should be set with angular contact bearings. Then again, you need to have the proper tools as well. Duplex bearings are not cheap, but it's the only way to go if you want real accuracy. I know what it costs to have a mold made, I can assure you even with the price of duplex bearings, the machine will equate to a big savings.