No, steel angle is not good, neither U or I profile. Square or rectangle box have much bigger moment of inertia than the before mentioned. 60x60 is minimum, 80x80 is best for normal machine. I prefer 100x100x3 or 4mm, as the bigger the better, but the design must be cleverer also. take a look at my first build, link in signature

No, steel is out of question. You need a mill for this. As a last resort the machine must be fixed gantry with moving bed and short Z. and very very rigid.

You will need 20 size square supported rails /Hiwin or similar/ with the longest bearing blocksblocks, xxxxxHA, not xxxxxCA

Basically with the money you say you have, you can buy the steel, the rails and ball screws and that's all. And thats my advice, make a small 300x300 or up to 400x400mm machine with fixed gantry, invest in proper bearings and ball screws. The slowly finish it whenever possible.