Rob,

There's no shame in buying an MD machine, or any starter machine. At that stage you need to be sure this hobby is for you and that is one of the ways to do it. You will also learn a lot about what is important and what factors are required for a really good machine.

Many, myself included, have considered pulleys and wires to slave the other side but in the end I've not seen many builds like that. In the end with wire tensioning, stretch and setting up concerns etc. most opt for just buying another screw and belt slaving or double motors etc. I would suggest you do the same. At 500-600mm wide it be well worth it to go to double screw.

Going back to your thoughts - if you put a single leadscrew at the top in the centre, it will have to be very high to clear the Z axis and stepper. The drop to the gantry will be so long that you will loose considerable stiffness with the mount.
If you put a pair to each side, but still high up then you will have long supports up from the bed to mount the steppers to (assuming raised gantry design), again loosing stiffness.
If you put a pair to each side, just above the bed then you have a fairly optimal position in that the driving force is similar in height to the cutter. The stepper mounts are short and simple. I think if you use side shields down each side of the bed (e.g. perspex), or dust extractor for wood you will be OK.
You can also run the screws even lower down, outside the gantry sides and run the screw level / lower than the bed, and this also works. I think it is not quite as good as the previous for cutter force offset, but this might easily be outweighed if the mounting is more convenient and can be made stiffer.