Much of the information that I would provide about maintenance and inspections is listed above by other awesome members here on Bigger pockets. Septic's are common in my area and I have overseen 20+ full system replacements, hundreds of repairs and thousands of inspections over the last decade. a system replacement in my area usually runs between 20k-40K. and a property will be red tagged quickly if you do not have an operational sewage system so don't take this lightly. MAKE SURE THAT YOU HAVE THE SEPTIC TANK AND LEACH FIELDS INSPECTED AND CERTIFIED IN GOOD WORKING ORDER BEFORE YOU PURCHASE.
Here is a few suggestions that I didn't see mentioned above.
1) Home warranty insurance companies have "ad on coverage" for septic systems. this will not cover everything but it might be worth looking into or asking the seller to provide a one year policy to help ease your fears.
2) Check with the local utilities and government office to determine if city sewer is available to connect. Many times utilities have been added and are available to hook up for substantially less than replacing a septic system in the case of a failure down the road.
3) Remove garbage disposals. if the property has garbage disposals your tenants will use them which add additional solid waste to your system.
4) Get a copy of the "site plan" for the septic as it was installed. this should be available at the local government office Here in CA it is the "environmental health dept". Be sure that your septic was permitted for the number of units and bedrooms that actually exist. Pay careful attention to the location of the septic system and leach fields in conjunction to the structure and the well (if applicable). If the property has a well make sure that the well is more than 100ft from septic and an all leach fields. You will not be able to obtain standard or FHA financing if well and septic are to close together. If the property has a well you should have it inspected also and make sure that a water sample is sent to the lab to determine that there is no e-coli in the water. (nasty but its much more common than you think). This could indicate that the sewage is leaching into the groundwater.
I hope this helps :)
Sarah Becker