Sufficient Grounds/Who to Consult

[amici53]

I have quite a few questions regarding a complicated situation. For five years I worked for the owner of an independent restaurant in Colorado as an administrative assistant/consultant/bookkeeper. As such, I have always been privy to the manner in which he conducts business. For the past two years, his business decisions, spending habits, and general behavior have been erratic. He ended up bordering on broke, and is in the process of trying to close an entity sale of the business.
Being aware of his financial situation, I continued to work while he waited for the buyer to come up with financing, and he received the initial funds. I am keenly aware of how stupid that was, so there is no need to dwell on it.
About ten days ago, I asked him for a check for two weeks pay, and he told me, clearly, that he did not need my services any longer and I could "send him an invoice".
I responded, in writing, that because he had chosen to dismiss me without notice or reason, I expected him to:
1. Return all furniture and equipment I had lent him for use in the business. I included an itemized list of all my property.
2. Pay me for the time owed, for which I included an invoice with a breakdown of duties performed, hours and rates.
3. Honor a signed separation agreement which stipulates that should either of us sever the business relationship for any reason, OR the business be sold I would receive a one time payment of $10,000.00 for services rendered.
I made it clear in my letter that I hoped the separation could be an amicable one, and that all I wanted was for him to make good on what he had promised. He has chosen not to respond to my letter, and will not return telephone calls or emails.
I am in possession of five years worth of his business records, and have knowledge of open accounts in the name of the corporation he is attempting to sell as an entity. These debts and liabilities would become the responsibility of the buyer, and I feel that the buyer should be aware of them.
I am in the process of writing a second, less congenial letter, in which I want to make clear my intent to pursue the matter, using all legal options available. While I do not wish to sound overtly threatening, I want to make it clear that by ignoring my requests, he could be setting himself up for headaches reaching far beyond the situation between him and me.
My questions at this time are these:
• How should the second letter be sent? Certified mail, registered mail, or hand delivered.
• Would it be advisable to send a copy of the letter to the people buying the business?
• If I receive no response, do I have sufficient grounds for a lawsuit?
• Can I litigate this matter without benefit of an attorney?
• What type of an attorney would I consult?
I apologize for the length of my post. I wanted to cover the matter as thoroughly as possible. Thank you for your time and any subsequent advice you may have.