Ok, here are my thoughts, for the $0.02.
1) The Z envelop of 18" is very, very tall, and is probably not going to be very stiff usinghome construction. Most folks have ~6" on large machines. Some software packages such as Vectric's Aspire and Cut3D can slice a job to make use of thinner pieces.
2) an 8mm pitch seems to be long; I've seen T5 and XL being used, at ~5mm
3) Unless the gantry is super stiff, pulling it by one side will cause it to rack. Some folks add a steel cable 'sliding know' to keep the gantry advancing straight, but on bigger machines most just add dual X drives.
4) Every pound of weight must be moved and accelerated. At 70 pounds, accelerating this at reasonable rates will take much more of your motor strength than the cutting forces.