its perfectly legal and widespread. kaplan, TPR, and many other companies do so. Certainly there are hundreds of individuals who do so from AMCAS thru secondaries and intetviews. the question of what constitutes appropriate editing is impossible to judge or discover.
However as someone who has done both private and school advising, the vast majority of applicants dont need it. At most an initial consultation in guidance and perhaps a review of PS, both of which which you can find free here. I turn away the vast majority of people who request my services as all they need was 15 minutes of guidance and sent to SDN , OPM or MSHQ (shameless plug). I think the only ones where most paid services might make some sense are busy nontrads who have schools, lives, families and just are too tight on time. Even then just an hour or two.
I've gotten to this point in my career that an atypical nontrad or vet will get my help pro bono as their unusual stories need help