No heat coming out and van overheating

It could be the thermostat or the water pump. Even if your heater core were plugged, the engine would not overheat from it, you would just have no heat. And if the thermostat were stuck closed, but the heater core was good, you would still get heat from heater while the engine was overheating. So it kind of points toward the water pump, unless neglect had allowed both the heater core to plug and the thermostat to stick closed. I mean neglect from not regularly changing the coolant every two years about.
Overheating can be as simple as a low coolant level, so you should check that, and check for leaks.
Before you go for the cost to replace the water pump, you should first have the system pressure tested, or have a compression test done to check the internal condition of the engine and to make sure you don't have a blown head gasket. Good luck.