Is it common to have a lawyer go over a Purchase Agreement?

The short answer is yes, but of course it depends on how big of a transaction it is.  An experienced attorney will no doubt spot issues and potential problems that you may not (it's not only what is in the agreement, but what isn't, and specifically what contingencies have not been proved for), but will cost money.  Doing it without an attorney is a risk, but, depending on the size of the transaction, you may feel the risk is worth it.