These vehicles have a "learning" mode that establishes some settings over time based on the drivers habits. You may need to "reset the ECU" so it will go back into the learning modes with you as the new driver. There is a seperate proceedure for clearing out the transmission memory as well.
I agree with lenore in that you don't have to replace the fuel filter unless you have particulate contamination in your fuel tank like chunks of rust or dirt and so on. You may want to look before wasting time and money.
If the vehicle specifies 93 octane, you might want to reconsider going to 89. That's more of a jump than lenore going to 91. Understand that the octane ratings are not "quality" ratings. They are a measure of resistance to ignition. 89 octane has less resistance to being ignited than 93 octane. Because of this, as the fuel is entering the cylinder, you will be more prone to pre-ignition, or spark knock than if you were to use 93. Which will mean less horsepower, (fuel is igniting before piston reaches top dead center), and rougher idles and lower mpg's. All that can add up to more money than if you were to just use the right octane. Can you get away with a lower octane? Probably. Will it damage your engine? Possibly. Given the used status of your purchase, the mileage and year, missing some maintenance, I would just stay with 93 for at least the first year of ownership. Get to know your new vehicle first. Then if you put in 89 and the change ok with you, then you at least know what's going on. But I'm puzzled at the fact that the fuel filter and maintenance history bother you, yet chincing out on the fuel is ok ?