Excel?! You need an accounting software. It depends how many properties you have to manage. Say you have 3 properties, Quickbooks will work just fine (about $100 at Staples or mail-order). Quickbooks has different levels but the basic one would work just fine.
If you have dozens of properties, you need more sophisticated sofware to track rent and repair history on each unit. There are so many software applications. TenantPro is not bad.
Keeping a file folder with written record for each unit is also recommended.
