OneCNC installs very easily (and cleanly) on your PC. You will need a good up to date PC now to run OneCNC since they updated the simulation engine: old AMD processors will not work, must be athlon64. Intel's PentiumIV work well. Graphics drivers for whatever card also need to be up to date. Both Nvidia and ATI video cards will work, again, a modern card should be used, but a 'gamer quality' card is adequate.
10BaseT is old hardware, but so long as you can successfully network with it, it should work okay. You would simply browse the network from your cnc and look for your nc files.
If you run an RS232 cable, OneCNC has its own communications software. Fairly basic to set up, same as any other RS232 system. There are communication protocols to set correctly on both the machine and in OneCNC, but you'll find help on this in the help file.
However, you use RS232 to download your nc file, not a dxf file, nor a solidworks model. If you want to download dxf files, you'd need to use your network connection. A word of caution: OneCNC does not make native dxf files, although it will export in that format, there are sometimes some incompatibilities. The whole point of getting a full blown cadcam is not to make dxf files, but rather to make nc files (containing gcode commands).