Hello,
I would suggest you meet a few independent parties and learn what you can from all of them, before finalising your negotiations. Accountants can be great, but some are more commercially minded than others. A tax consultant may be able to offer suggestions on the most tax efficient method of compensating your investor, while a good financial advisor may be able to share experiences of other similar transactions, and possibly assist you with negotiations etc.
I concur with the opinion of cremeegg and Steven above, a return in the double digits would be appropriate for this type of investment - however, whether it's 12% or 18% is open to negotiation and depends on the specifics of the business.
While you have suggested that there is little risk, the fact is that most small privately owned businesses actually have quite considerable risk given they are often quite vulnerable to competitive forces etc. Industry sectors play their part, time established, management team, whether they sell goods & services for cash or are forced to offer trade terms to their customers, whether they have a strong brand, sole agencies, a stable trade, steady margin etc. Also, from an investors point of view, they cannot easily get their money back if they need it - unlike say a quoted company where they might be able to sell their shares on a stock exchange. I could go on....
