Scales are great to know if you want to improvise and make your own melodies, many might disagree with me on this, but I’m a scale-fanatic ;D
What kind of music are you into?
Edit: if you play the guitar you should go with a diatonic and a rack, so you can play both at once!
Double edit: SP20 is probably a great harp to get! Other good choices to fair prices would be Suzukis Pro-Masters, Hohners Blues Harps or a Lee Oskar harmonica. Anything that costs more than 20 bucks and less than 60 is good for starters.