I have just built Nelson's house and started on his tasks. Based on the forum detailing all the quests that need to be be done before getting Krabappel Apartment I figured I had time to save for it, and given I had surplus cash I purchased the post office.
My thoughts are this - if you have the cash might as well get the post office when you can. Even though its a little disappointing with no characters etc. every one will want it in their Sprigfield to be complete, so get it when you can. It's only simpsons money - not donuts!