if you can weld then making a cnc out of steel would be an advantage as welds are stronger than fixing with bolts an nuts, if you do use ally then people tend to bolt them, I think it personal preference, I know you said you have a table to use as a base so you could weld extra supports to brace an sniffen it.

As to regards to designing your cnc in a cad program before you get to building, I’m currently doing it and I’m getting all my demensions off the net, ie rail sizes and block diemensions, so the information is out there just obvious you have to draw each part in your cad program, I’m not sure if you can find them as a open source design and just put them into your design, I’m using solidworks so don’t know if I could attach what I have to help you out design wise. As soon as I’m finished with mine I’ll let you have a look lol

The linear rails are better as they take more load and has massive advantage over the supported rails, there are videos that compare the two if you want to see why, the main thing that made me choose linear rails is there more accurate and have a lot less play the the blocks, again I’m still learning but the price difference it’ll be silly to go cheap on a key parts like rails, ball screws an spindle as there not easy to upgrade in the future.

I hope this helps and looking forward to seeing what you come up with, I’m still along way from getting my build log off the ground