The steppers you currently have are rated at 140oz/in best I can tell. So they will be under-powered for heavier machining. But the new ones your looking at are way overkill, you'll get no benefit from going to such a high torque stepper with these machines. Around the 230-280oz/in is definitely more than enough grunt to do whatever you need with the machine. My large 6090 weighing in at 300kgs uses 380oz/in steppers, and these are more than adequate for frequent and heavy machining!
Ideally, as low an inductance as possible is preferable, the formula to work out the required power supply is the square root of the inductance, x 32, to give the DCV required. The steppers you linked to are 3.8mH in unipolar mode, so the standard 24v supply with that kit I wouldn't consider good enough. A good 48VDC power supply, with a G540, and some 2.3mH steppers would be a perfect combination.
The new spindle you linked to I have also not heard good things about, most of the comments on that model seem to indicate a lack of power and torque, with frequent failure rates, especially as it's a brushed motor. A proper spindle and VFD would be preferable, but you may be stuck with one of the brushed motors instead as you've linked to due to the size of the machine.
cheers, Ian
It's rumoured that everytime someone buys a TB6560 based board, an engineer cries!