For a homeowner who doesn't want to spend much money the hand file will work if whey don't let them get too bad. In my shop working for a customers I use an angle grinder. I just don't know how an 21 in. blade can wear to 12 inches, something I would like to see.
I see that the choke and oil has been covered. When you say it starts hard what is it doing? Turns over very slow? You might want to check to see if the fuel pump is holding pressure seeing it starts with starting fluid. There are many things that can cause hard starting?
Buy a cheap mower and you will have to replace it more often. Buy a good brand mower and pay a little more for it with good maintenance it will last for years. My Ferris is going on 20+ years old and still runs and mowes as good as the day I bought it.