Do you have the amplifiers/drives that were there originally? If not then the spindle may be a problem depending on the whether its encoder or resolver and if encoder what type.
The axis motors being DC should be easier, there are plenty of DC drives available from low end right up to industrial quality. My friend actually has some mid range type drives for sale but sadly he is in the UK so probably not an option.
If you can find info on the spindle encoder then I may be able to suggest a drive.
Hood

I have the original axis and spindle drives for the both machines, as well as a couple of spares but from what I remember each machine had one bad axis drive-I just don't know which one is which.  My initial plan was to find out which (if any) spare drives worked and fix whatever other issues arose but the machine is a tape reader and runs on an older control which I have no experience with my plan switched to retrofit, which is where this thread started.

If I can use the stock servo motors that would be great, it (sort of) saves money, and increases the machines potential.  I'm just skeptical about using the old boards because I don't really know which (if any) boards are actually functional anymore.

But both spindle drives should be functional-there was never any mention of them being broken.
Now that I think about it there may have been a 3rd machine because I do have an entire extra set of motors as well as the spare boards...

That is good that you have the spindle drives as they would have been the hardest to match up. AC servo motors are often best matched with the drives they were meant to work with, some drives can be set up for different motors but a lot of them you can only use specific motors as there are not any options to set custom motor parameters in them.
 DC motors however are easy as almost any DC drive will work as long as it is capable of delivering the current and voltages that the motor wants.
Encoders may have to be swapped but usually DC motors just have 5v square wave encoders so hopefully that will be what yours are.
Hood

Is there an easy way to check they type of encoder?  I have a lot of documentation for these machines that I could sort through.

Just want to be clear, if these are the square wave encoders I should be able to use the spindle drive, all stock motors, and pretty much any 9A/150+/-V DC servo driver and 3/4/5/6(?) axis breakout board, and either the DSPMC, KAnalog/KFlop or CSMIO/IP-A board and run this off of Mach3?

Any recommendations on DC Servo drivers/breakout boards?

The encoder or resolver on the spindle motor wont matter as you will be pairing it with the drive. The only issue you would have is if you wanted spindle speed or rigid tapping, if the encoder was not suitable for the controller (csmio/dspmc etc) then you would have to fit another encoder somewhere els on the spindle.

There are more Step/Dir drives for DC available in the low to mid range market than there are analogue but I think AMC make some decent ones.
Hood

Z Axis:

DC Servo Motor Model 10M Type A06B-0651-B212
Torque cont 12 Nm
Amp cont 12 A
165 V
1500 rpm

Spindle encoder:

Optical Shaft Encoder
DC 5V
1024 counts/turn

Sounds like the spindle encoder will be ttl but there is a possibility it is not. If you could find a part number on it you may be able to find a data sheet.

Regarding the axis motors, as said basically any DC drive should do as long as its capable of the voltage/current the motors require. There are two options for you however, do you go the cheaper way and get the more hobby orientated drives which will be Step/Dir input or do you go a bit more industrial style and get analogue capable, personally I would look for the second option purely because I like the fact that the analogue controllers always know where they are due to encoder feedback to them.
Suppose there is a third option and that is to look out for second hand DC drives, they often appear on eBay and you can get some very good deals as most people are wanting AC drives nowadays.

Hood

Saving money by not having to buy new motors and knowing that I can retain the positioning feedback make me more willing to put up some extra cash towards better drives (A-M-C?).

This is all a bit more technical than I am used to so excuse me if this question sounds silly but what is the difference between the KFlop/KAnalog system and the CSMIO/IP-A system?
I have seen some info on threads about KFlop and they seem to have excellent technical support, but  haven't seen much about CSMIO.  They are both in the same price range as far as I can tell so I'm not really leaning in either direction yet.

http://www.ebay.com/itm/MIYANO-BNC-34-CNC-TAMAGAWA-OPTICAL-SHAFT-ENCODER-TS1508-N91-TSI508N91-/161009529900?pt=LH_DefaultDomain_0&hash=item257cea742c

This is the type of encoder I am dealing with.  I can't find any data sheets online so I will have to sort through the heap of documentation that came with the machines.