I'm not sure I understand your question completely...
Are you wanting to go as fast as possible USING the current components you have, with 4-500 more bucks to spend?
Or is the 4-500 for the entire machine?
Or are you asking what is the best control I can put together with 4-500 bucks? (and wil use what I have already IF it makes sense)
Or what is the best control for these motors if I have 4-500 bucks to spend?
Next, to really give you a good answer, I need to know more about what size/type machine you have in mind? What do you think you will be making with it?
Finally, since you emphasize speed; how much resolution are you willing to give up in exchange for speed?
Because a machie with 100 steps per inch will be able to "humanly possible" go much faster than one using 2000 steps per inch...
Thank you in advance for your answers,