Samgaz:

Channel isn't very stiff and certainly not flat. Welding in braces etc will cause heat and twist etc. So unless you intend machining the Gantry flat and true then I'd suggets you look at better materials that won't need stiffening by welding.

Your Z axis idea with Channel is disaster waiting to bite you. Like Clive mentions the practical side of bolting etc means it won't work but again Channel just isn't flat enough or good enough. Z axis IS THE MOST IMPORTANT part of the machine.

25mm ballscrews are too large for machine this size and will actually lower the performance and increase the costs in ways you haven't realised yet.
The extra interia of the Screw and Ballnut means you'll need motors with higher torque. This will mean using Nema 34 Motors.
Nema 34 motors spin much slower than smaller nema 23 motors for the same amount of voltage. So this means you need higher voltage to get the same or similar RPM.
In practise this means using drives which offer much higher voltage capabilty. While 60-70Vdc will be fine for nema 23 it will cripple performance of nema 34 and the Larger the motor size the worse it will get. So often larger Nema 34 Motors use drives that run at mains voltage levels.

If you try to run tham at same voltage would nema 23 then your RPM will be so slow the machine is crippled.

NorgMonster: This also applies to you and it's the only comment for you I'm making on this thread so start your own if haven't already.

It's common mistake that Bigger is better and offers upgradabilty. It DOESNT. What it does is Cost more and lowers performance.
Nema 23 motor matched with correct size and Pitch ballscrew run with correct voltage on decent quality drive will run rings around and cost much less than Nema 34 motor running on wrong voltage and connected to large ballscrew.

In your case Norgmonster with R&P then you'll probably need nema 34 motors because of the much lower efficiency of R&P. But again if you don't correctly size and match components then peformance will suffer and money will be wasted.
R&P machine requires careful thought towards pinion size and Reduction ratio used along with Motor/Drive/voltage selection.

Both of you are going about this all wrong and you will Both end up wasting money, time and probably resutling in less than great machine unless you STOP buying without knowing what's required and Design the machine before starting. Fag packet drawings while ok for some things just make this CNC building a disater waiting to happen unless you have plenty of experience which you two clearly don't.!!

Stop, take Step back look and learn then design. Then look again before buying any thing.!! . . . If in doubt ask.!!