Regarding your design, how will you attach the rails to the extrusion? Just nuts? Do you have any plans for keeping the rails precisely flat?
I’m thinking about doing similar things to my Nomad and I was thinking of extrusion but ruled it out for lack of precision. My thought was to buy a pre-milled 15mm or 20mm EN AW-7021 plate, bolt the linear rails to it and stick it where the Nomad’s X-axis rods are with a bracket or by putting some beefy threads into the plate.
In case it helps, I’m using the CNC720 on my Nomad and I love it 
You might find my thread helpful, I compare controllers and drivers.
Since you’re cost-constrained, it might also make sense to keep the steppers that came with the Shapeoko. If you’re in Germany I could probably send you one of the adapter PCBs I made for the Nomad, though I’d have to think some more about how useful it is to you. My first board might work out of the box, depending on which stepper drivers you use. The later board is nicer and more compact but also very focused on the ESP32 and TMC5160.