As you have the pro version of MBAM i would leave it on since it provide's a beneficial extra layer of security, it has jumped in and saved my butt a few time's when avast has not detected anything.
As for the configuration of avast it is best to leave at default setting's which provide's a good balance between speed and performance until you are more understanding of all the avast feature's.
With an i3 processor your system should not be that old so i would think that it shouldn't need cleaning, is normal for the fan to kick in and run if the system has been on for a while anyway, do you have laptop or desktop?