You could probably get away with a 5HP spindle/ You can get a Chinese one for less than $1000, and probably save much more than that simplifying the design?My problem is I have the 20HP spindle... I think we could get away with a 10 or 12HP but really I have no clue.
But not spending 6,000$ on a new spindle and use what we have seems like the way to go.
How much material will you be removing in one pass?