Don't re-invent the wheel. Go with a common design.
1. Look at the CNCRouterParts kits. A welded base with an extrusion top is a very popular way to go.
2. Look at the Joe's Hybrid design. Very similar to the CNCRouterParts kit in the method of building and the flexibility.
3. Mechmate machine. This is an all steel, all welded machine.
The Mechmate is arguably the best bang for the buck machine but the Joe's or CNCRouterPart's machines can be put together in a couple of weekends if you're focused and can devote the necessary 20-30 hours. Mechmate is probably 3-4x the amount of work. No matter what, you should budget around $5,000-8,000 for the project. Between all the extras, software, bits, the spindle, ect ect ect that's a realistic budget.