Beginner questions

So im looking into switching to linuxcnc for 2 new machines, one lathe and one mill. I previously have been running mach3 but it lacks cutter comp for one. 

Im a bit at a loss with what i need to get started been looking around at the Mesa cards not getting any wiser.

I get that you can get away with using parallel ports with linuxcnc but the manual says its only for slow speeds, what is the limitation there? 

For my lathe i need 3axis + the usual estops, mpg, limits, homing etc but on top of this i want to create a panel like you would find in a real machine and im assuming this would bring the i/o need to more then whats available with 2 parallel ports.

If one wants higher speeds ie ditch the parallel port is it a external motion controller one needs or just something that uses another interface and linuxcnc still spits out the individual steps?

Think i will stop there for now

In default situations, LPT port will be enough.
All depends on the power of your PC. I thing 16000 pulses/sec you can receive.
More over if your machines worked on mach3, then they definatly will work on linuxcnc

If you were running your machines with Mach3 through a parallel port then Linuxcnc should be able to run them just the same. If you are using an external motion controller with Mach3, you will need to either replace that with an external step generator compatible with Linuxcnc or control the drives more directly in the case of something like analog servos. Linuxcnc doesn't use eternal motion controllers the way Mach3 can.

Where Mach3 sends essentially chunks of g-code to the motion controller, and lets the motion controller determine its own steprates.  Linuxcnc on the other hand only outputs the current position or velocity command and handles all of the motion control internally.

Linuxcnc was first developed for controlling analog servo systems, controlling step pulses was added later. And because of this, creating step pulses in Linuxcnc is usually handled more like controlling a servo. Linuxcnc either sends a current velocity command or current position command (updated at the the servo-thread rate) and reads back the position feedback (either by reading a position sent from the step-generator or servo drive, or by counting encoder pulses.) And in the case of a velocity commanded system uses the position feedback to adjust the velocity command output to match the commanded position to the feedback position. It may sound needlessly complex but it works surprisingly well. A Linuxcnc compatible step-generator simply generates pulses at the rate commanded, counts the number of pulses generated and then feeds the resulting pulse count back to Linuxcnc.