A few questions to consider, and budget is number one because it will affect and may limit everything else.
1. What is your budget?
2. What cutting area do you need? Remember, you may be locked on your Y-axis, but you can index your X-axis. In other words, on a 2ft x 3ft cutting area, you may be limited to 2ft wide, but you can cut a part 4ft long by using dowel pins or some other method to index it (cut then shift part over).
3. What will the machine be made of? MDF, plywood, structural c-channel, 80/20 extruded aluminum, etc.
4. What tools do you have access to? A welder, drill press, jig saw, table saw, etc.
4. ACME threaded rods work great, but once you get to 4ft or longer, you might want to consider something else, such as rack-and-pinion.
After you have the main frame and moving parts figured out, don't forget:
2. Power Supply
5. Mach 3 or equivalent
6. Cut 2D or equivalent (convert to g-code)
Actually the site in the link has some great info, and here is a link to a budget file that can be opened in Excel:
CNC Router Budget LINK