If keeping the original drives it would be the IP-A you would want, it is the analogue command one, the IP-S is Step/Dir and your drives will almost certainly not be Step/Dir.
As for price, well not really sure, you would definitely need the IP-A, you would also need the Enc module if wanting Rigid Tapping and likely you would also want the MPG module so that you can easily integrate the MPG and buttons on your panel.
What else you would need on the machine I don't know, maybe need a spindle encoder, although there will likely be one there, apart from a computer then I think more or less what I have mentioned, plus time of course, should about do it.
Hood