A capacitor won't help with the start-up unless you also use a timer & relay to delay starting the blower until after the PSU is on and your cap's have charged.
Easier is to add inductance in series with the blower. On mine I've recycled a big inductor from a dead PSU.
The diode in the diagram is absolutely necessary if you add inductance (and a good idea even if you don't). Use a diode that can handle 100 Amp surges, preferably a Schottky diode.
The cap in the picture is to cut down on the electrically noise produced by the brushes. Use mylar or polyester or film or ceramic caps and have a close to the blower as possible. Different values handle different frequency noise, so use a combination of a couple. I used 10 pF, 1 nF, 100 nF, 10 uF.
