If I want to sell a used mobile home and the buyer will pay in monthly installments, is a purchase agreement whatI need?

You should have some writing as to the understanding for which you are selling the vehicle. How detailed you want to go depends on whether you are giving him title right away. In order to properly protect yourself you should have a lien filed on the vehicle with motor vehicles so that the new owner cannot sell the mobile home without paying off the lien. This will remain a lien on title with DMV until he pays it off. Otherwise you have no security on the vehicle. The other option is to get a promissory note with a breakdown of the installment payments that he must make until the debt is paid. This will allow you to sue him for the balance in the event he does not pay. However, this will be unsecured and the vehicle is not secured as collateral.  The third option is a rental agreement to which the payments he makes are put towards the balance until such time he pays, at which time you sign the car over to him. The problem with this is that as the owner you will be responsible in the event of an accident. While you don't necesarily need and attorney to do this, you may want to review any dosuments you prepare with one to ensure that you protect yourself .