Usually, elevator pit pumps are only for when the sprinkler goes off.
How was the building built without a pump if the elevator pit was below the water table?
When you say it is below the water table, how far below is it?
If the bottom of the elevator pit is significantly below the top of the water table, then you could have some significant problems.
If it is, you are probably looking at a system where your pump could be working a substantial amount of time.
I would suggest getting some advice from a civil/geological engineer about groundwater flows in the area. There are probably some on this website who might give you guidance.
Size the pump to pump out at least equal the flow rate in. Duplex the pump system so if one is down, the other can operate or if the flow in peaks, at least you should be able to not flood the pit.
Good luck