Hello and here I go

[JAZZCNC]

Ye nice colouring.. . Lol

Sorry to be negative but I'm just trying to save you some pain and money.

Being completely honest with you this size machine is not a wise choice for a first time build.? . . . Thou That does not mean it's not do-able because it most certainly is.!

Large machines change the game quite considerably when it come to linear motion and it becomes very important to success that the right choices are made. First time builders while they may have the skills required to accomplish the build don't always see or realise the importance of some of the things needed for success.!

The choice of linear rails and linear motion system like R&P etc become very important. Both these things are important to any successful build large or small but over a certain length, approx 1500mm, the choice and implementation raises the stakes and chances of failure rise greatly. This is why avoiding the weak designs like Angle iron/bearing linear rails becomes so important.
But the bigger issue over this length is the choice of linear motion becomes a much less simple affair.?
The usual simple choices like lead or ballscrews are suddenly very complicated.! The alternatives like R&P while more suited come with there own set of needs and things that have to be correct for success. . . . It's these that I was trying to point out before.

Now to answer some of your questions and hopefully set you down the right road for your needs and wallet.!

Seen has you can weld then it will be far better machine and easier to build from steel so that's the route I'd suggest every time.
Without seeing the full design then the use of C channel I can't fully comment on but IME box section works much better and far stronger. C channel is rarely flat or square on the inside and using the outside to have a decent level of strength requires a much thicker wall thickness which then makes it costly.

You only need 1x rail 2x bearings per side. 25mm supported round rail will be fine for this length and about the cheapest way to do it correctly.

To be honest the building of the frame is the easy part it's this next bit where it gets funky.!!. . . . . R&P or Ballscrews.?

Ball-screws are far far better than R&P but at this size they need a special attention and implementation is needed.! . . . Namely they need a rotating ballnut design.
This will mostly be beyond the first time builder to DIY build them selfs and far to costly to buy from a ball-screw supplier. The other option is to have another person make the rotating ball-nut assembly, Jonathan on here has a good design and I'm sure will gladly make for the right amount of beer tokens.!!

R&P then becomes the next choice has that doesn't have some of the issues long ball-screws have IE Whip but like I pointed out before they do have there own set of needs. Again Namely they need correct gearing ratio's, careful and accurate alignment of rack, usually larger motors due to inefficiency, larger drives and PSU to run the bigger motors.!! It also requires more maintenance than ballscrews and requires that the rack is kept clean and free from debris. . . Try to skimp and it will bite you with binding, racking, stalling and generally frustrate the hell out of you.!!

So has you can see not simple or ideal for a first build.!! . . . If you take the cheap route then be prepared to be frustrated and constantly chasing problems and unless you have a good grasp on what a CNC machine needs and the traps involved then not something I'd recommend at this size.

Do it correctly with the right components and careful selection of methods then YES it's very much do-able for a first time, easy NO but very possible.

Good luck.

PS The prices from china I don't know but the duty's will be VAT on the rated value on the shipping paper work plus a shipping handling fee of about £10. Often the supplier will put a lower value on the paper work so the Vat payed will be lower.! IE My last bought was £460 and I payed £20 vat. .