MakerBot Print is our newest print-prepration software, which supports native CAD files and STL assemblies,
allows you to interact with all your printers via the Cloud, and many other exciting new features.
300x200 bed - sure it'll be fine. Keep in mind its a 12V heatbed so you will need a beefy power supply (i.e. 12V 40A) and some very thick power wiring, like 10AWG or so.
Single Z axis - I wouldn't skimp on this, use a dual Z axis IMO. It's much more stable especially for larger format printers. If your 'long hanging' side is the 300mm side then definitely don't use a single Z. I have two HCEvos with dual Z axis and alignment isn't an issue. It holds well.
NEMA17 motor for Z (and X/Y) - most will be ok, not the tiny ones but the ones that tend to be 40-48mm thick will have ample torque.
TMC2100 - I haven't used these but I know people like them for their high microstep interpolation to make things quiet. That said I don't have any issues using simple A4988s with my printers.
Igus Bearings - I use these on my X axis along with 10mm carbon fibre rods and it's fine. I use standard LM10/LM12 linear bearings for my Y and Z rods, which are all steel. There is some data on the Igus web site about the performance and longevity of their bushings on various different types of rods. Just don't use steel bearings on an aluminium rod as it'll cut it up.
Cutting carbon fibre - use hand tools, like a hacksaw, with a particulate dust mask. Don't cut all from one side, go "around" a few cuts at a time. They're actually relatively easy to cut compared to solid steel (especially hardened steel!)
Belts - go the steel belted belts, there's little reason to cheap out and go for fibreglass reinforced belts. The HCEvo is a CoreXY mechanism and one of the few pitfalls of this design is that it's quite picky about belt tension and stretch.
Arduino Due - DUEs are a good cheap option to get 32 bit processing on an HCEvo. I am using an Arduino Due with an AZSMZ "Smart RAMPS" board (available from Aliexpress and eBay). I like it, it's basically like using an Arduino Mega + RAMPS, except you need a different AZSMZ display (also available at a decent price from above).
About the DUEs themselves, many have a bug where they need the reset button pressed before they'll boot. The fix is to find an "R3-E" model of the DUE, or do some tiny soldering to add a resistor to the board.
24V PSU on steppers - increasing the voltage to the steppers ends up increasing their torque. They'll step the same speed, just with more grunt.
Weight of X gantry on speed/accuracy - it does a lot, look at the "This is Y" video on YouTube by Tech2C for a demonstraion.
Torque vs print speed - I guess, but 3D printing isn't particularly stressful on stepper motor torque, the extruder motor being one big exception. You will need a beefier extruder motor if you up the speed a lot, and/or use a geared extruder.
Using light X gantry rods is a way to save a lot of weight. I have run my printer at 200mm/s doing an enlarged Ultimaker Robot for shits and giggles, and it coped just fine. I doubt the extruder motor enjoyed it though. A more sane printing speed for me is around 80-120mm/sec.