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A moving gantry is going to present real problems with cutting metal. Building from aluminum can be problematic if you are wanting to cut metal. Using a router is going to cause real problems cutting metal. A machine this size is going to be very difficult to DIY to cut metal, even aluminum.
You will likely be better off building a small desktop router for PCB work, though many have built large format machines that are capable of PCB work. The large format DIY builds are typically routers designed to cut wood & plastics and sacrifice some resolution and accuracy for higher feed rates. If you absolutely have to cut metal and plastic on the same machine, you should consider going with a laser or plasma cutter instead of a spindle for cutting the metals. With a laser/plasma cutter there are no rigidity issues like there will be with a spindle. If you build a very rigid aluminum machine, you might expect to make light cuts in aluminum with the right spindle and the right feed rate, but you will certainly not have much luck with cutting steel.
To answer your questions in order:
1) The forces required exceed the capabilities of any typical aluminum build by a lot. The main problem is not the torque on the motors, but the deflection of the machine itself. A typical metal cutting machine out weighs it's wood cutting counterpart by a couple orders of magnitude for any given work envelope. This weight is requisite because large steel sections are needed to prevent deflection.
2) A lot depends on the machine specifications and the type motors used. For instance steppers experience their highest torque at low RPM and the torque drops off significantly as the RPM increases. Servo motors have fairly constant torque over their entire RPM range. As a first approximation you should take the total mass that you will be moving and determine what your maximum travel rate will be then calculate how fast you need to be able to accelerate to or from that speed. When you reach a number, double it.
3) In a moving gantry you will likely have the long axis set up as your "Y-Axis". With a 1.4m X-Axis travel, your gantry will likely be at least 1.5m wide; if you decide to use ball screws, your build will almost certainly require two (one on each side). "Sagging" is not really the issue (though it can be problematic), the real issue is "whipping". For your 2.4m Y-Axis you will likely need to look at 20mm to 40mm ball screws, or consider a rotating nut design.
4. I have been researching DIY lasers, but there is a lot to learn. You can order the "CO2 Laser Tubes" fairly easily off ebay and other sites, but making that laser tube into a cutting head appears to be rather involved.
Yes, we all want 0.0001" accuracy in an 8ft x 4ft work envelope with 1000ipm rapids for $2000 that we can fold up against the garage wall when the wife gets home ;-) Sadly, like most things there are trade-offs. You can build for speed, accuracy, size, cost or weight, pick three, the other two will be out of your control.I'd like to keep this machine precise despite it's size, as I'd use it for pretty much anything from cutting small printed circuit boards, to making large custom shaped sheets of wood or mdf.
The best advice I can give you is to start with a small desk-top router accurate enough for your PCB work and small wood/plastic parts. You will learn enough from it to make informed decisions about a larger format machine. You should be able to build something like John's (Microcarve) "A4" machine for ~$500 to $1500 (depending on steppers, controllers, software, rails, lead screws, etc, etc), and that money will be WELL SPENT. John sells his "A4" machines for ~$400 (w/o steppers, controller(s), software or power supply), so you should be able to source the parts for less than $400. The steppers/controller(s) and Power Supply can be as cheap or expensive as you want. Building a 4ft x 8ft format machine for your first build, you will make bigger mistakes than $1500 can fix several times before you finish the project, so consider a desktop build an inexpensive lesson.
You might consider reading some of the build threads in the woodworking machine forum here @ the CNCzone. I know you think you want to build a metal cutting machine, but I don't think you are quite ready yet.