We bought a 2005 Jeep Grand Cherokee Laredo 4X4.

You need a new mechanic. First of all, heater cores spring leaks all the time, just ask any GM owner, and that does not damage the water pump or thermostat. The water pump can leak on its own, and thermostats do stick every once in a while but in general they are very trouble-free. There is nothing you can do to cause them to fail, including a leaking heater core.

You didn't say which engine you have but $500.00 for a water pump seems way too high.

Cracked block? Without even seeing it? A cracked block is so extremely rare, even after severe overheating. That is the last thing any professional would even think about looking for. I've run into one in my entire life, and I caused it in the '70s by leaving it sit in winter with only water in the cooling system, no antifreeze. I'm confident your block is not cracked. Your mechanic, ... I'm not so sure.

The typical progression of events would be the heater core was leaking, the coolant level got low enough to prevent circulation so the engine overheated, it wasn't noticed so the cylinder head(s) warped to the point the head gasket couldn't seal. The white smoke from the tail pipe is from burning coolant. It gets into the cylinders through the leaking head gasket, burns, and goes out the tail pipe.

"He said no, it had to be a cracked block and that it would be very costly to see if it was the seals due to all of the censors in the newer models."

There are very inexpensive tests to see where the coolant is going. A cooling system pressure test takes a few minutes. A cylinder leakage test takes longer but will show up as air bubbles going into the radiator or reservoir. There's also a "sniffer" test that checks for combustion gases in the cooling system. None of those require disassembling the engine. As for all those sensors on newer models, those have been around since the mid '80s. That's what we're trained to work on, but you don't do anything with them. When you replace a head gasket, the sensors come off with the cylinder head. There's no special procedures or things you have to do. You either need a new mechanic or you need him to learn how to communicate better. Either way, you need a second opinion.

I'd try a few shorter trips before I took it 200 miles. As long as the coolant keeps going out the tail pipe, or combustion gases go into the radiator, you can get by as long as you keep an eye on the coolant level. It doesn't take much of a leak to make a big cloud of smoke so you could very well make it 200 miles without adding any coolant.

What you don't want is a leak that lets coolant run into the engine oil. Antifreeze will melt the outer layer of the bearings. That won't happen right away, but over a short period of time. Once they start to disintegrate, you'll have bigger problems. Check the oil level before you start the trip, then again when you get there. If the level went up, the oil should be changed. The oil is normally changed anyway after doing a head gasket because little pieces of debris, and stray coolant can get into the passages and into the oil. If coolant is in the oil, it will be a milkshake-brown color.

You can also try loosening the radiator cap one click. That will prevent pressure from building up in the cooling system. That pressure helps push coolant through the leak into the engine. That pressure can also reduce the amount of combustion gases that go into the radiator. Those gases, if the leak is fast enough, can prevent the thermostat from opening which will lead to overheating. Thermostats open in response to hot liquid, not hot air. If the temperature gauge reads in the normal range, leave the radiator cap loose. That may reduce the cloud of smoke you leave. At slow speeds there might be coolant splashing from the loose cap but that is less likely to happen at higher speeds where natural airflow keeps the radiator and engine cooler.