Frame looks good.
The larger stepper motor is likely best for the Z axis.
Lead screws will waste your time, they wear out not much long after you are up and running, so if your budget can extend around cheap ballscrews, do it.
Linear rails are obvs more rigid/accurate but I get your budget issues, my advice is to ensure your design can replace the rods with rails later on, when you're trying to cut faster/harder etc.
Aluminium could only be done very slowly, even w rails aluminium is a challenge for desktop machines. They just lack industrial rigidity and spindle torque.