Albert,
I agree with Dean completely. There's so much to learn and in several different areas of subject matter all at once that deciding to postpone building anything until you've gained more knowledge is a sensible way forward, not in any way a failure. Once you do start building you'll be more confident that the choices you make are the right ones.

Having to move from Mac to PC is annoying but it sounds like you have little choice on that one. Learning F360 will keep you out of mischief for a while anyway.

The build logs on this site are a wealth of knowledge on different approaches to how you can build a machine, the aluminium v steel choice for the base frame being a big decision you will need to make. The aluminium you already have may still be used in the gantry. Don't be too daunted by the very smart appearance of some of the results on the forum. If the pieces are straight and flat and the holes are in the right places then the rest of it is largely cosmetic!

I was a bit daunted at trying to weld but for me in the wilds of Western Australia the cost of aluminium is prohibitive and the choice was made by my wallet. However the purchase of a cheap welder means I now have a very useful tool that can be used for many other projects as well and I'm glad I took that route. It's actually a lot easier than you think to make a joint that works, as long as you aren't expecting to build anything seriously structural like a road bridge or a grain silo.

Kit