The carraiges are the expensive part and the rail is cheaper so if you can get some compatible rail in a useful length then do that (sorry, some cash required here).
Use the unsupported rail for Z, the three large carraiges for the Y (one on top rail and 2 on the lower rail where the forces are higher). Use the 4 small carraiges on the long X axis that the gantry rides on, with one spare.

Use the profiles for gantry and bed with the plate for gantry sides and any spare plate for a spoil board / work area

Save up for ballscrews plus the rest.

Unfortunately CNC can’t be done cheaply but the above would get a fun starter machine. Make a nice job and maybe get more for it to put towards mk2? Best of luck with it !