linuxcnc with lubuntu

You can use older laptops, namely the IBM thinkpad T40 or T41 or T42 as they do have a parallel port, they have decent and stable latency once you disable some useless stuff in the bios and they are built like a tank. Also the X60 works perfectly but it needs a docking station if you want parallel port.
In case you go the ethernet route, you would need one of the Mesa cards like 7i92 or even better 7i76E if you are using steppers, that would greatly expand the type of laptops you could use.
Avoid new laptops, they are useless for anything CNC as they tend to have way to many power saving features.
As a side note, it is irrational to use a laptop for a machine that is not portable, but i still tend to use them a lot.
From the desktop side, any older Dell enterprise version would do just fine, especially the optiplex series with pentium 4 or pentium d or core and core2 types of processors. At 30 to 80 Euro a piece, it is a bargain.
To start using Linuxcnc you can download 2 versions of it, the
www.linuxcnc.org/linuxcnc-2.7-wheezy.iso
That it used with a parallel port or Mesa or Pico systems hardware, or
www.linuxcnc.org/testing-stretch-rtpreem...uspace-amd64-r10.iso
That works with all of the above and ethernet.
You can download and follow the instructions for making a bootable USB stick and you can test them as much as you like even with no hardware attached.