100% agree with Lee (No I'm not sucking up so you lot can shut up before you say it.!! ) Let this be a lesson to others on how to go about starting a CNC build.! . . . Well done Sir A-Star +.!

I do have some suggestions for you thou. Other than turning the motors outward which is a must do.!

Put some ends on the gantry it will greatly stiffen it up. I would also think about lowering the ballscrews has they are vunerable up there and some form of protection from chips etc.
This can be achieved by making the gantry narrower so it's just extends past the sides of the machine. The gantry ends then extend down past the top rail 3-5mm away from the steel and on the inside of the ballscrew. The gantry sides can then be bolt to the gantry and the bearing plates making gantry slightly narrower and a very strong structure.
This then allows you to place the ballscrews lower down out the firing line of chips protected by the top rail and damage from dropped materail etc.
The gantry side being on the inside of the screw makes alignment and setup of the ballscrew much easier. More importantly it narrows the restriction from the moving part the gantry and allows you to cover the screws easier with covers for protection. If your not sure what I mean ask and I can provide examples.

Also think about extending the top rails past the ends of the machine.? If done at both ends this can gain you back some cutting area and full access to work area by placing gantry in space using a short cantilever and at other end allow gantry to pass the front of machine so materail to be clamped on the end and edge machined or over height material.
I suggested this to Eddy at time he was designing and he didn't take up on it to only later end up cutting lumps out of his bed so could edge machine material ie: dove tails etc.

Only other change to frame I would make is to remove the back plate on the axis which moves acreoss the gantry. It's not required, it adds weight and very little strength. Can make the building much harder has the mating edges need to be perfectly 90deg otherwise it can twist and bend the bearing plates. Profiled linear bearings don't tolerate this very well and will bind.
Nice strong Drop down bracket will work just has well be much lighter, cheaper and easier to build.

Electrics then the only thing I would change is the BOB. For get the cheap chinese stuff you'll only regret it. I wouldn't use DIYCNC stuff either if your going with Smooth stepper. Honestly there really is only one BOB worth using with the ESS and that is PMDX 126.
All the others will at some time give you trouble or restrict the machine performance. Pay the extra at the begining and you won't regret I promise you that, which can't be said for the others. Trust me on this I've used most if not all of them.

What I've suggested are just that suggestions and what you have drawn so far would work great but some of those things I've suggested I think you'd find would probably be on your bucket list for Mk2.!!

Well done and if you build with the same careful attention then you'll have nice machine.