I got my hand slapped the other week - you need to re-size your pictures to 800 x 600 pixels - any of the cheap picture editing programs will bo it.
If not, download a freeware version from the internet.
Before you can get good advice, you really need to make your own mind up about what result you want to see.
My own system is as simple as it gets. A lathe/mill driven by home installed stepper motors. No measuring device or feedback, I don't bother with limit switches. My speeds are very modest. ( That said, it does the job - and I can keep my eye on it when I am running)
I am quite sure - in fact I know - that other contributors have very fast moving, large machines, they need all the safeties (and in an industrial set-up they are probably mandatory)
I am familiar with Bridgeport mills - and the older ones tend to be large, slowmoving machines - very accurate etc. is it for industrial use, or the home workshop environment.
You have probably read the Mach 3 stuff over and over again, so you will know what Mach 3 can do. I would suggest that, for ease of use, you hold on until the Smooth Stepper is available ( coming soon from an outlet near you
), because the lads that have tried the pre production stuff say it is the bees knees. I anticipate - although I have not seen one, that this will give all the inputs and outputs necessary (and a few more besides) to wire to your lathe/mill in any configuration you want, and the only connection to your computer will be a USB lead - rather than having to mess about with the old LPT1 lead and a breakout board (which is limited and then you need other boards etc etc)
In the meantime, I would suggest you contact the makers of the motors, you say they are nearly new, and ask them what drivers and power supply systems (and more important, control systems) they recommend for them. They made them, they should know, and quite frankly, they are nothing to do with Mach3 (I don't mean that rudely). They are a bit of the machine - and that varies in all our different applications. As "Ssheidenr" said - if they are working now, something must drive them - and if they are CNC already, that probably Mach can deal with them.
If they are steppers - fine, I know about them, but I know nothing about servo motors and linear feedback instruments. The thing I do know is that Mach can deal with both easily (and does).
The spindle can be dealt with equally simply - I had to install a new three phase motor, but after that a commercial (Omron) inverter solved my control problem and Mach 3 was able to interface with that simply and easily. You might find that an inverter ( you may already have one) is fitted and you can connect speed control from Mach to your machine without any further interface.
As I say - I think the answer is up to you - how do you see it progressing.
After that, anything we can do to help.