The 6.2 isn't a terrible engine, but it's far from the best. They were known for head gasket issues. Some versions were also known for cracks in the block around the main bearing web. There really is no guarantee that it will or won't have either problem.
The temperature gauge getting into the upper quadrant could be indicative of head or head gasket failure, letting exhaust or cylinder pressure into the cooling system. But it could be something as simple as a thermostat that doesn't quite pop open when it should, or something as simple as air in the system, explaining the low coolant indicator, and could interfere with thermostat operation.
Is the gauge marked with numbers, and if so, what are the numbers? Some gauges read as low as 100, others read as high as 280. Upper quadrant of a 240 limit could be 210, which would be normal. Upper quadrant of a 280 limit could be 240-250, which would be very bad.
Knowing the range is important. If the gauge reads 100-260, one-quarter travel would be 40 degrees roughly, so three-quarter travel would be 120 degrees, or 220. If the gauge reads 140-280, one-quarter travel would be 35 degrees, and three-quarter travel would be 105 degrees, or 245 degrees.
212 is normal boiling point, which can be raised by 3 degrees for every 1 psi in the cooling system. Most systems are equipped with 9-12 psi caps, but check this against the factory spec. The radiator cap seal can weaken over time and cause problems. Not all caps made to fit will be the right pressure rating either.
The reluctance to fire when cranking could be as simple as a bad glow plug controller, this was fairly common on the 6.2. Lots of people have been known to simply wire a toggle switch to the relay. However, if you did not let the glow plugs cycle before cranking, that is likely why.
The glow plug indicator is there to caution you to let the glow plugs do their thing, they are there to pre-heat the cylinder to assist with cold starts, something diesels are not predisposed to doing easily. With a glow-plug-equipped diesel, you turn on the switch, but pause to let the glow plug controller will cycle the glow plugs a few times to pre-heat the cylinder. Then crank. Starts will be easier. Otherwise, you risk damaging the starter.
The smoking leak could be a rear main oil seal or transmission seal leaking on the exhaust crossover pipe.
Overall, sounds like you found a fairly good egg with which you need to learn to get used to the nuances of a diesel. Just make sure the temperature / possible low coolant issue isn't anything serious. The only cause for concern I see is the smoking leak, which sounds to me like fluids dripping on the exhaust. If it doesn't have lots of rust, could be a good buy. NOTE: Diesels cannot generate vacuum, which means your power brake system is likely hydroboost, interconnected with the power steering system. Some may be equipped with a vacuum pump for other accessories.
One other thing, if it is not already equipped with a fuel/water separator, and you choose to buy it, have one installed as soon as possible. Water in fuel is pretty common, and it is death to a diesel. A water separator will allow you to drain water out of the bottom of the fuel filter housing harmlessly away from the engine.