I am looking to purchase a used H1 for a daily driver. However I have noticed that most early models have there engines replaced. Why is this, and what other problems could I expect with buying a used H1?

lots of h1's crack a cylinder due to overheating, there are aftermarket crossover kits to solve this. maintenance is key in a hummer, i recommend one that has seen the shop every year. look for a 98-99 for a better off roader at a good price. stay away from the earlier ones if possible. 99 gets tt4 and has abs. other common problems are pumps, abs modulators, and idler arms

It's not that they are cracking a cylinder because of overheating it's because of blowing a head gasket. The 6.2's and still the 6.5's are susceptable to overheating in the military. The 6.5's are better suited for military and non military use.
They will crack between the cylinders with Extremely excessive heat but excessive heat is the bane of these engines. The water crossover kits only permits water to get to the thermostat in a less restrictive state IMHO, Ok maybe a little water will crossover.:)