As an air conditioning repair service, we have dealt with many faulty AC systems. The reason for the majority of problems is poorly estimated capacity and placement.
There are many things to consider when moving into a new home. Purchasing a good AC is one of the most important ones. A lot of people underestimate that, and end up regretting it. The reason most people fail is, because you need to get an accurate calculation of the capacity you need, before buying anything.