Something simple that you could possibly attach a document and it would alert when maintenance is about to fall off.
I suggest ManageEngine ServiceDesk Plus. It's simple and easy to use. You can manage complete asset inventory - hardware, software, software licenses, warranty, asset depreciation and so on. You can add attachments too. On license expiry, you will receive notifications through email as well as SMS.
I have found a warranty tracking software “SPICEWORKS” that automatically manage computer warranties and gives you an alert to remember your warranty expiration.
Hope this might be helpful to you.