the graphtech seems to be a good choice...
as far as needing 4x8 sheets since you will only have a 24 inch plotter is a over sight on your part, most vinyl comes in 24 inch wide rolls
you can do things like tile the letters etc... and what are the chances that the copy will be taller than 24 inches??? most signs have more than one row of copy and more times than not it is less than 24 inches.....
as far as the minimal investment with huge profit part you are being mislead,
this is no gravy train, and if you do not have good design skills you wont go far, it is like any other business you will need to work hard to get established
and maintain a customer base, good luck //chopper