Linux CNC Newby basic questions...

1. No
2. Not sure what you mean but probably not relevant since Linuxcnc does not work with USB controller
3. PC choice will depend on what interface you are planning to use (Parallel port or Ethernet connection to an FPGA board like the MESA products or EtherCAT )

Use a parallel port with a simple breakout board (cheapest solution but somewhat restricted regarding step frequency and input/output options (eg no analog) )
Or use a compatible Ethernet board which is more capable and less demanding on the PC but also more expensive.

Note that, unlike with Mach3, LinuxCNC IS the controller so usually those 'ethernet controller' boards sold for use with MACH3 will NOT work with linuxcnc.