Ion,
chances are probably good that your control voltages for that machine is 24V, (look at the solinoids for your air valves). Plus older solinoids can sometimes pull upto 0.5 amps per coil. Having said that, if you do the retrofit with just PP boards, you going to have to go through Relays. Your other option is
some of the modbus boards, some of them offer secondary "Relay" boards as well.
The best option as Hood said, is PLC's since you get a device designed to work day in, and day out in an industrial enviroment. If you IO needs are small less than 8in and 6 out, grab a DL05 they are around 100 bucks or so, if more than that but less than 20 in, and 16 out, grab a DL06 (they sell a model that has all the outpus already as relays, it runs around 220 or so. If you dont have many rungs of logic in the plc, then the Demo software that you can download for free if your program is less than 1000 bytes, more than that you have to buy the programming software, and it runs about 300. I use Automation Direct since they are economical, BUT, you can pick what ever you want to use/afford, just make sure you can get your hands on the programming software for what you choose.
Gear Shifting: Look at what the logic sequence is on your Air for the Gear positions, and look at the logic sequence for the "Change gear" (if any, I suspect it just changes from one state to the other without a Transition state). What you can do, is watch your Current Spindle Speed in mach3, and depending on that value, send out the changes to your gears once it crosses the change point in your speed. You can run that from the Macropump, or Brains.
scott