Whilst it's true that as it stands that machine will cut plywood at a reasonable speed it would improve it no end to have two ballscrews. I'm confident most people make a CNC router then ultimately end up using it for all sorts of things they didn't expect, so why risk compromising it later. True it's not cheap to have a second ballscrew, but as a percentage of the whole machine it's not so bad?

If you do decide to stick with one ballscrew then space the linear bearing blocks out as much as possible to compensate. Ultimately your material removal rate will be proportional to the spacing of those bearings, along with a number of other things of course.

Also another vote for linearmotionbearings2008. When the time comes send him a message asking for an invoice for the exact parts you need since that's generally a little cheaper than the eBay listings.

The stiffness of your gantry will be greatly improved by simply adding an aluminium plate (10mm-1/2" ish) thick to the back of the gantry. Bolt the extrusion to it all the way along. It will virtually eliminate bending of the extrusion thus reducing bending of the rails.