Pavlos, good luck with your project. I cannot give any technical advice regarding such a large machine. My own DIY build was a benchtop model in aluminum.
What I can say is that a machine like you're describing is a big undertaking. It will take a lot of time and much more money than you initially think. Once it's up and running you will probably discover that you undersized the motors, or need new electronics, or should have done something else differently.
You can't expect to get everything right the first time. If you get 50% of it right, it's probably a success - but you may feel the need to keep tinkering and improving, and that is more money and time you're not spending generating income.
I don't want to discourage you. I have two or three other machines in my head that I would love to build someday. These CNC machines are a great project. You will learn so much, and have a knowledge of your machine that you'll never get from a commercial unit. But the commercial units also come with support, diagnostics, documentation, etc. If you truly want to use this for your business, you may find that it makes sense to invest a little more money now and save yourself a LOT of time and frustration later.
I don't know what your CAD/CAM/CNC background is. If this is all new to you, you might consider building one of the proven benchtop designs first. Learn to use the software, learn about the electronics and motors, about the different options for linear motion and spindles. Then you can tackle the big, 5 axis you machine you dream about. Also, don't forget the software to run such a machine will be tremendously expensive. It alone could cost several thousand dollars.
Good luck, please keep this thread updated with your progress. There are so many great builds on this site. Anything you can dream of can be built, but as you're aware, it is a big undertaking.