I do think didgee may have a point?

Students who are willing to pay for editing and tutoring and assistive services outside of the scope of what their school can or will offer are students who are looking for that extra edge to get a great grade. They are likely to be very competitive.
It would be likely that they do not qualify for any disability accommodations at their school and many schools have restrictions on the amount of free tutoring offered through their student/academic centers.
[Students who are registered through disability centers almost always qualify for either unlimited or a much larger amount of tutoring, however]

But this means that any student paying you for services would expect to get at least the level that a tutor through their school would provide, or even better.

So that means having a working familiarity with, in the least [and having at your disposal] most recent editions of:
Strunk and White's Elements of Style
the APA publication manual
the MLA style manual [though THE OWL @purdue is a great resource for this and you probably don't really need the manual]

If you haven't studied formally or had experience with research papers [which I'm not actually clear on? Maybe you have just not in an academic setting, I can't tell, sorry :/ ], you need to get practice in reading and correcting citations if you are going to be editing papers. Many professors, in particular for upperlevel courses, can be extremely strict on this.

Some are more lenient, but specialized topic courses for particular majors? I've had professors who will have your head if you misplace a semi-colon and because of that if you miss a couple of those, you will feel the rage of an angry student who got a good five to ten points off a major paper because of a misplaced punctuation mark or two.

I'm not saying you CAN'T do it, but there is a lot to editing research papers. It wasn't clear to me if you had thought of all these things, so just trying to be helpful in mentioning them.

I have a lot of experience doing research writing using both MLA and APA for bio, psych and english lit courses. Switching back and forth between mla and apa will drive a person completely bonkers!

ETA:
http://www.apastyle.org/ is a great source for everything apa style and they have a blog around there somewhere that even addresses some of the weirder issues that can come up that aren't addressed with clarity in the manual.

Something you could try to find a topic:
Look for syllabi that universities have posted online in a particular subject?
Pick a few keywords that interest you and plug it into a search engine [ERIC. pubmed, medscape]. I am majorly into psych [clinical and research actually haha], medicine and genetics... if you see me in chat feel free to hit me up if you would like to bounce some ideas off?

... I say that just as much for you as for me.
Now that I'm not in school anymore i feel like I don't ever get to talk about nerdy stuff anymore