For thin material, I hear that it could be as fast as 400ipm to cut properly, so you need to decide what you will be cutting and how fast the plasma needs to go.
On a table that big, for routing wood, I would consider 200ipm the minimum speed as I can't stand waiting 30-40 secs for the machine to travel from one end to the other.
200ipm is still going to take 15 secs for full travel.
700rpm on a 50" screw will probably whip and shake, but I use rack and pinion so I am not 100%sure.
You should be able to search for router builds on the zone and see what people use for similar sized machines.
I think that 700rpm from a motor that size is asking a lot but look at the torque curve and see what kind of torque the motor has a 700rpm.