Yes some times best to sit back and re-think before jumping in.

Ask your self what's the main purpose of the machine going to be.? Be realistic and honest with your self. If it's mostly wood then you don't need go to the trouble of expensive plate.
Your original design sat on nice flat stable surface will work ok for most woods, plastics etc. If it's aluminium then yes the strength and accurecy is needed so the costs rise.

Regards cutting rails then it's always something to avoid really purely on being a waste. The actual cutting is easy. All you need is angle grinder with 1mm thin cutting discs and you'll whiz thru them.

Been said many many times on forum with lots of examples and this is think towards Steel if you want cheap and strong. But slight warning here.!! IME trying to make something from the scrap bin work, like what you found, always ends in a compromise of some kind and in the grand scheme often doesn't save you much money long term so be weary of this appealing route.!! (That said Steel offcuts etc are ok just avoid structures for bases etc)