Gary first let me say forget work bee, shepoko, etc, or cheap Chinese machines because none of them can repeatably cut aluminum to the accuracy that you require.

Like Kit says building a machine the size you want to cut with the accuracy you want is no small thing and requires a good strong design that is very well built. To build at this level you'll need other machines or have access to someone who can manufacture parts for you. It's also not something that can be done cheaply or corners cut.

My advice is don't buy a cheap machine because you will be disappointed and will waste money long term if try to upgrade, etc. Likewise don't undertake to build a machine until you have done plenty of research into which design will suit your needs best and also exactly what's involved in building it. Also, be very honest with your self about your ability to achieve this and the true cost of building.

Many people have achieved building machines like what you require but none are done cheaply and all of them that succeeded did so because they planned well and didn't cut corners.

If you decide to build then start a build thread straight away and ask all your questions in that one place. This way you can go back and find any suggestions made weeks or months ago. Don't be afraid to ask what you may think are Dumb questions because we will have been asked them before and don't mind repeating our selves.