First a little background.
I started working as an in-house (read: underpaid) translator at a Japanese owned/run translation after moving to Tokyo a few years.
After realizing that I could do (and often did) exactly what the freelance translators who worked with that company did, and that they made loads more money than my terrible salary at the time, I decided to leave the company and go freelance.
So for the past year/year and a half I have been working as a freelance agency translator. Basically I sign a contract with a couple of agencies, they send me work, I send it back, and they pay me and withhold 10% of my payments for taxes, most of which I get back in my tax return anyway. For tax/legal purposes it seems that going through these agencies and being paid the way I do, is essentially handled the same as pulling a salary from a company. No special hoops to jump through, no registering as a business, etc.
Thing is, I am now fairly successful with the agencies, and get more work than I can handle. So, now I am starting to think about taking on direct clients (clients who work directly with me, cutting out the agency) because where I make somewhere between 6-10 yen per Japanese character on a translation, most of the agencies I work with are actually charging the customers anywhere from 20 to 40 yen, and I want a bigger piece of the pie

So, finally to my question, if anyone has experience with this, or with something similar:
In order to operate legally (a lot of customers will be businesses, so I can't do things "under the table" and cut out the taxman/government):
1. Do I need to register as a business?
1b. If so what would be the best route to go? As I understand it, kojin jigyou don't really require much more than filling out a paper saying you are a kojin jigyou, but would there be any benefit to goin YKK or Kaushiki, etc.?
2. Obviously this will change how taxes are handled, because the clients will pay me the full sum, and I will have to handle reporting taxes, etc myself, but are there any special procedures for this that I need to know about or can I still just fill this in with any normal income on my kakuteishinkoku?
2b. If there are special procedures and stuff should I contact a tax professional?
2c. If I should contact a professional, does anyone have any recommendations? I don't really know anyone in this business, so I figure a recommendation would be at least slightly more reliable than opening up the yellow pages to the appropriate section and playing eeny meeny miney mo.

Any information anybody would be willing to share would be greatly appreciated.