I suggest digging the hole deep enough to set the basin on some 1.5" thick "patio blocks" for support.
Then I would drill a ring of holes around the bottom of the basin, just above the radius at the bottom. The idea is to drill enough holes so the pump can run continuously as long as there is water to pump. That will reduce the cycling of the pump. You don't want the pump to be running 15 seconds on and 15 seconds off because the water can't get into the sump as fast as the pump can pump.
If you have enough holes the pump will operate continuously until the water table at the sump is reduced to within a couple of inches of the holes in the basin. Then it will run only as often as necessary to keep the water level down.
I would put at least one 5/8" hole for each GPM of the pump at the discharge head of the installation. For example, if the pump is rated at 20 GPM at 10 ft head and the discharge is about 10 ft above the pump, then put in 20 holes equally spaced around the bottom. If you use 1/2" holes, put in 30 holes for 20 GPM. More holes is always better if you have enough room without destroying the strength of the basin.
If one row would put the holes closer than about 2.5" spacing, then put a second row about 1.5" above the first row so they won't have to be so close together.
Put some larger stones around the bottom to allow the water to get access to the holes, and then fill the space around the rest of the tank with smaller stone, like 3/4" or 1/2".
Set the pump on the bottom of the tank.