FWIW, Re: Brush Springs... if it's not broke, don't fix it.. these thing take a lot of punishment and are a PITA to replace. The brushes also last a for long time without performance issues.
Smell, these motors typically have a slight petroleum/ozone odor, motor/armature are not typical replacement items.
I suspect the odor is from the brush contact on the armature which creates a small spark /heat which causes on odor due to petroleum residue on the armature being heated.
Hope this helps.