whats the main focus of the build? i assume youre more into light gaming than into photo/video editing
also you will prob not be able to reuse the ram as laptop ram is of the SODIMM formfactor while normal desktop memory is bigger
also do you know if you can rip out the SSD without too much trouble? afaik macbooks have most components soldered onto the board
if you can reuse it, it would be really good though