here are things you need to consider..
techno sold almost all size of router you can imagine..
when you read someone retrofit, or talking about one, you have to see its ranging from 8x6 inches up to 60x120 inches..
yours is larger then most desktop they sold..
the mechanical parts are almost never wear out..
if oyu put on too ""small"" motors it still works, but it will be unefficient..
I understand your hesitation, but you need to take some time to see trough this..
about 600 this gecko can be in your shop, you hook up on exist motors and half our later you can run the router..
but you have a 4 feet wide gantry.. a nema 23 motor drive it, because it has ballscrew with no friction.. it will be very much on limit..
while using individual drives, if something going awry you can simply replace one drive to a larger..
youre new for this, but many folks made it from same situation than you....
I just try to encourage you, get some insight before you go out to spending your money..
between servo and stepper the price difference 1.5-2 times generally..
however you can get good deal on servo.. and it wont be so much more..