moving gantery CNC sturdiness

[JAZZCNC]

And the base matirial i was thinking of was steel rectangle tubes (50*100mm with wall thickness 3.2mm) mainly cause it is massive and should vibrate less and it is can fit into my budget

No it's not massive and is barely just thick enough if your wanting a strong machine with low resonance. Even 5mm wall is only just good enough and will only give average vibration dampening. 3mm wall resonates like a tin can.

And the first question that was on my mind is how to connect all the parts of the frame?
I think i will not be welding because i dont think i have the skills to make it precise enough
So the alternative is having the steel cut precisly at the store, connecting with bolts that way i have option to compencate most errors with shims and a file
(If you have any thoughts or tips for me it will be very helpfull)

Precision doesn't come from welding it comes from setup and alignment, strength comes from welding. You don't need to be a welder to build a strong CNC machine and a grinder hides all sins. Many good CNC machines have been built and welded by people who had never touched a welder before so don't be put off from welding.
That said bolting as it's place because it does allow easy adjustments, like wise it easily gets knocked out of adjustment and isn't always easy to keep the precision you worked hard to achieve when setting up.
My advise is to do a mixture of both and I've said it many times, build in as much adjustment as possible into the design, then look again to see how you can add more adjustment. Because like you not many people have the equipment to machine parts to high or tight tolerences so you need other ways to achieve this and building in adjustments allows you to do this. However you have to think about how you can lock those adjustments so they hold fast.
It can take days, weeks or months to fine tune a machine and it's not something you want to be doing very often.

The second thought i wanted to run by you is : i was thinking of geting the two ribs under the linear rails from thicker steel (3.2mm 50×100mm tube) and the other two ribs of the frame from thinner (40×40mm 1.5mm thick steel profile) below the first two ribs

Nope waste of money and time. As I explained above 3.2mm is not thick and 40x40 x 1.5mm is about as strong as wet spaghetti.